This title will no doubt appear novel to many persons, and is, perhaps, open to adverse criticism, but it serves to unite in one group all of the mountains in the eastern half of North America. A cordillera, as usually defined, consists of two or more mountain chains associated geographically, but not necessarily of the same age. On the Atlantic border of the continent we have an example of such a family of mountains. The Atlantic mountains, although comprising ranges, systems, etc., of widely different ages, are all geologically old, and have resulted from upheavals along two generally parallel and slightly overlapping northeast and southwest belts adjacent to the Atlantic Ocean. The growth of this group of mountains is believed to have been from the north southward, and several periods of upheaval have been recognised.

The two main divisions or chains referred to are separated by the valley of the St. Lawrence. The mountains at the north are known as the Laurentides or Laurentian Highlands, and those at the south comprise the mountains of New Brunswick and Maine, the White Mountains of New Hampshire, the Green Mountains of Vermont, the Adirondack Mountains of northeastern New York, and the Appalachians. The most convenient method of reviewing the characteristics and histories of these several uplifts is to begin with the Appalachians, which are at the same time the most important and best known, and consider them in their order from south to north.

The Appalachian Mountains.—This beautiful and frequently exceedingly picturesque series of long, narrow ridges separated one from another by trough-like valleys, constitutes a mountain system some 900 miles long and 50 to 130 or more miles wide (Fig. 15). The truly mountainous portion in its widest part, in western North Carolina and eastern Tennessee, is about 70 miles across, but a portion of the adjacent plateau on the west partakes of the same structural features and is a part of the Appalachian uplift. The system is considered as extending from the Hudson southward to central Alabama and central Georgia. At the north its terminus is indefinite, as it merges with the highlands to the east of the Hudson and with the Berkshire Hills of Massachusetts, which in turn are not strictly separable from the Green Mountains of Vermont. At the south, the system ends somewhat abruptly where the crystalline rocks comprising its southern terminus pass beneath the soft sediments of the coastal plain. The eastern border of the system is well defined by its junction with the Piedmont plateau, but on the west it merges through a series of lessening folds with the plateaus and plains of the eastern border of the interior continental basin. The Alleghany plateau, which skirts the western border of what is usually recognised as the Appalachian Mountains, but which is really its moderately disturbed border, extends from the Hudson to Alabama, and in its various portions is known by distinct names. Its northern extension overlooking the Hudson forms the Catskill Mountains; farther south it becomes locally the Alleghany plateau, and still farther south the Cumberland plateau. Separating the bold eastern escarpment of this series of plateaus from the generally higher mountains to the eastward lies the great Appalachian Valley, which under various names extends from the Hudson to central Alabama. This important and highly fruitful valley is underlaid to a great extent by thick bedded limestones and soft shales, and owes its existence to erosion and largely to the removal of limestone in solution.

The Appalachians are nowhere lofty, and only approach the characteristics of great mountains in their southern portion. The culminating summit is Mount Mitchell, in western North Carolina, which has an elevation of 6,711 feet. Roan Mountain, 27 miles to the northward of Mount Mitchell, rises 6,287 feet above the sea. In the neighbouring Unaka and Great Smoky Mountains, to the southwest and west of Mount Mitchell, there are many boldly rounded domes ranging in height from 5,000 to over 6,000 feet Northward of the highly picturesque southern Appalachians, the system decreases in height and is really a deeply dissected plateau, as will be shown later, in which the long, even-crested ridges have a general elevation of 4,000 feet in Virginia and about 2,000 feet in Pennsylvania and New Jersey. At its northern extension in New York it decreases still more in height, and is surpassed in elevation by the plateau on the west, there represented by the Catskill Mountains, the highest portion of which is 3,660 feet above the sea.

The characteristic structural feature of the Appalachians is the presence of a great series of up and down folds, or anticlinals and synclinals as geologists term them, which run in a nearly parallel northeast and southwest direction, but in Pennsylvania especially exhibit many broad curves in their general course. These folds are similar to the waves that may be produced in a heavy rug or carpet by pressing against one of its margins. The rocks have been thrown into a series of great wrinkles which are not continuous throughout the length of the system, but as one dies out another takes its place. The folds overlap at the ends or are arranged en échelon. The longer axes of the folds are seldom horizontal, but have usually a gentle pitch; for this reason one end of a fold frequently passes beneath the surface, while the other end is exposed to erosion. Another characteristic is that the anticlinals, as a rule, are steep on their western margins, and slope more gently on their eastern flanks, or are unsymmetrical. The overturning of the folds where most pronounced has led to the breaking of the rocks on the west side of an upward wrinkle where the descending limb of an anticlinal is sharply bent in order to pass into the ascending limb of the adjacent synclinal. These breaks or faults in certain instances form thrust planes along which one portion of a series of beds has been carried westward, sometimes for several miles, over another portion of the same series. This highly characteristic system of unsymmetrical folds, passing at times, and especially in Tennessee and Alabama, into great thrust planes, is accounted for on the general theory that there has been lateral pressure or a tangential thrust, which has forced the strata into a series of elongated arches, in much the same manner as in the case of a rug, as above suggested, one margin of which has been forced by lateral pressure towards its central part.

The rocks composing the greater portion of the Appalachians are stratified marine sediment such as sandstone, shale, limestone, etc., which were laid down one on another until a great depth was attained, corresponding, as we may fancy, to a pile of rugs, the original thickness in Pennsylvania being about 40,000 feet. Lateral pressure resulting, as it is believed, from the cooling and consequent contraction of the earth's highly heated interior, and the movement of the cool and rigid crust in order to keep in contact with the shrinking mass beneath, has led to the folding and occasional breaking of the rocks, which at the same time were elevated above the sea. A crushing together or folding of the rocks similar to that which has taken place along the central part of the Atlantic border of North America, as is well known, has occurred also in many other regions, and the Appalachians may be taken as the type of a class of mountains, sometimes termed corrugated mountains, which includes the Alps and Pyrenees, the Coast Range of California, etc. For convenience we may speak of such mountains as being of the Appalachian type.

Had the folding in the Appalachian region gone on without erosion, the surface would to-day be a series of great, elongated arches or upward folds, rising in many instances 5,000 or more feet above the intervening valleys, and where breaks or faults occur their upraised borders would stand as mighty cliffs, in some localities a mile or more high. The central part of the region with this strange topography had there been no erosion would, perhaps, be fully as prominent as the Himalayan Mountains are at present. No sooner, however, were the Appalachian Mountains upraised above the sea than the destructive agencies of the atmosphere began their attacks upon them. The rocks were shattered by changes of temperature, and at times at least crumbled by the freezing of absorbed water and also underwent chemical changes which softened and disintegrated them. The rains beat upon them, and streams flowing to the sea cut channels and carried away the material forming the land. These processes of disintegration and erosion have been in progress since islands and continents first appeared on the earth, and every mountain range now giving diversity to the surface of the land represents the net result of elevation over denudation. The Appalachians are not an exception, but a typical illustration of this general law. The great folds of which they are composed have been truncated by erosion and the surfaces thus produced, etched, as it were, by the action of the air, rain, and by streams, so as to leave the edges of the more resistant layers in relief.

One conspicuous result in this general process of erosion is due to the fact that the folded strata consist in many instances of alternating hard, or insoluble and soft, or readily soluble layers. Where resistant layers underlaid by soft, or readily soluble strata formed the summits of arches they have in many instances been broken in the process of folding or cut through by streams flowing down their flanks and the weak beds beneath exposed. After this stage was reached the erosion of the upward folds went on more rapidly than the removal of rock from the compressed downward folds, so that what is structurally a ridge became a valley: while the bordering troughs or synclinals floored with hard layers were left in relief as ridges or table-lands. The anticlinal ridges have thus been transformed into topographic valleys and the original synclinal troughs left in relief as plateaus and ridges.

The characteristics in the present topography of the Appalachians just considered are but a minor portion of the great changes that have resulted from erosion. The history of the system has not been the same in this connection throughout, but retains evidences of successive upward movements with long periods of erosion intervening which have produced certain striking differences in its northern and southern portions. These differences are so well marked that it is convenient to divide the system into two portions, termed the northern Appalachians and southern Appalachians. The most conspicuous difference between the two is shown by the direction of flow of the larger rivers. At the north, the principal rivers—the Delaware, Susquehanna, Potomac, and James—rise well to the west of the mountains and flow southeast athwart the numerous folds, and after crossing the Piedmont plateau and coastal plain discharge into the Atlantic. At the south, however, the rivers, particularly New River and the Tennessee, rise on the eastern border of the Appalachians and flow westward, cutting through the Alleghany plateau, and are tributary to the Mississippi and the Gulf of Mexico. The somewhat arbitrary dividing line between these two provinces follows the divide to the north of New River, or in a general way, as has been stated by C. W. Hayes, is marked by a line drawn from the most easterly point of Kentucky southeastward to Cape Fear, on the Atlantic coast.

The fact that several large rivers rising to the northwest of the northern Appalachians flow directly across or through the numerous ridges composing the system in deep, narrow valleys, and the similar behaviour of the streams rising on the eastern border of the southern Appalachians, but flowing westward, are among the most interesting features of the entire region. Why is it that the mountains have not formed a divide or water-parting so as to force all of the streams having their sources on its west side to take what would seem the easier course, and to flow to the Gulf of Mexico, and cause the waters falling on its eastern slopes to flow to the Atlantic? The answer to this apparently puzzling question has been furnished by Davis, Willis, Hayes, Campbell, and others, who have shown that the mountains were not raised all at once, but experienced upward movements at widely separated intervals, with intervening periods of rest during which the elevations previously produced were more or less completely planed away by erosion. During one of these intervals the north Appalachians more especially were worn down to approximately sea-level and a gently sloping plain produced across which the larger rivers flowed to the Atlantic. This peneplain was later upraised into a plateau and its downward inclination towards the east increased. The streams were thus given greater energy and began again to deepen their channels. They held their right of way acquired on the featureless erosion plain and cut deep trenches through the edges of the hard layers which crossed their courses. At the same time lateral branches were developed which followed the outcrops of the less resistant beds and eroded them away so as to leave the hard beds in bold relief. As the edges of the more resistant beds became more and more prominent the eastward-flowing streams cut deeper and deeper into them. The even summits of the ridges, one of the most striking features in the beautiful scenery of the Appalachians, still mark the position of the elevated erosion plain.

In the southern Appalachian the old erosion plain formed nearly at sea-level was tilted gently westward, and the streams flowing over its surface given initial courses in that direction, which were maintained as they deepened their channels, and on account of increased energy originating from the upraising of the region drained by them, developed lateral branches, as is the case of the more northern streams just referred to, and the process of carving away the land to sea-level was again renewed.

Portions of the original upland or mountain mass left unconsumed during the long period of planation, which reduced most of the region nearly to sea-level, still remain in eastern Tennessee, western North Carolina, and northern Georgia, and form the highest and most picturesque portion of the Appalachians.

After the upraised peneplain from which the long, even-crested ridges of the Appalachians were produced by the excavation of the bordering valleys had been deeply dissected and the valleys broadened, another upward movement took place and the streams again deepened their valleys. This is the stage in which we now find the mountains. The crests of the ridges, characteristically displayed in eastern Pennsylvania, are portions of the first peneplain of which a definite record is preserved, while the broad valleys with sharply cut channels in their bottoms represent the much less complete second stage of planation.

The two ancient peneplains referred to above, the histories of which are recorded in the topography, have received definite names in order that they may be readily designated. The older and higher one is termed the Schooley peneplain[2] on account of the preservation of a typical portion at Schooley Mountain in New Jersey, while the lower one, represented by the broad valley through which flows the Shenandoah River, Virginia, is known as the Shenandoah peneplain. A generalized profile in a northwest and southeast direction through a portion of the Appalachians is shown in the following diagram, which will serve to make more definite the description just given. The highest summits in the diagram represent portions of the Schooley peneplain; if the depressions could be refilled the surface of the great plateau formed by the elevation of this plain would be restored. The bottoms of the broad depressions represent the Shenandoah peneplain, which is sharply trenched by the modern river channels.

The Appalachians abound in beautiful scenery, but, except about a few of the very highest domes and ridges, have little of the stern ruggedness which is typical of truly great mountains. Their countless valleys are now mostly cleared of their primitive forests and under cultivation. To a large extent also even the steep hillsides are tilled. The larger trees which formerly grew on the mountains have nearly all been felled, and where the land is not suitable for cultivation their place is taken by a dense second growth. Under the mild, humid climate that prevails, more especially from the vicinity of the Susquehanna River southward, the rocks are deeply disintegrated and decayed, and even steep mountainsides are mantled with soil and rock débris. It is the excess of disintegration and decay over erosion which gives to the mountains their usually flowing outlines and pleasingly picturesque rather than rugged scenery. The valleys still retain much of the material washed from the uplands, and are deeply floored with rich soil. The characteristic colours of this decayed rock-waste are many shades of red and yellow, which harmonize in a most artistic manner with the prevailing green of the plant-covered uplands and abandoned fields. These red and yellow soils, particularly about the bases of the higher summits of the southern Appalachians, afford abundant crops of cotton, corn (maize), and tobacco.

The Mountains of New England, New York, New Brunswick, etc.—The picturesque Berkshire Hills, in the western portion of Massachusetts, have rounded and flowing outlines and a generally subdued relief. The more prominent of these greatly eroded remnants of what was once a mountain range rise but 2,000 to 3,500 feet above the sea. No satisfactory boundary between these hills of gneiss, schist, and allied metamorphic rocks, and the others of the same general character in the neighbouring portions of New York and New Jersey, has been determined. So far as the relief is concerned, and so far also as the complex geological history has been deciphered, there seems no good reason for separating the Berkshire Hills from the Appalachian Mountains. It is convenient, however, to consider the Appalachians as terminating at the Hudson. The Berkshire Hills when traced northward merge with a region of similar topography which unites them with the Green Mountains of Vermont, the highest summit of which, Mount Mansfield, attains an elevation of 4,364 feet above the sea. To the east of the Green Mountains are situated the still higher and more rugged White Mountains of New Hampshire, which culminate in Mount Washington. This widely known and greatly admired peak has an elevation of 6,293 feet, and, next to Mount Mitchell in the southern Appalachians, is the highest mountain on the eastern side of the continent to the south of the newly discovered group of peaks near Hudson Strait. Associated with Mount Washington are at least 15 peaks, each of which is over 5,000 feet high, and a still larger number of lesser summits which exceed 4,000 feet in elevation above the sea. The remarkable natural beauties of the Green and White Mountains, the ease with which they can be reached by means of railroads, and the numerous summer hotels, and hospitable farmhouses interspersed among them, make this, the most mountainous portion of New England, a favourite region for summer rest and recreation. The Green and White Mountains are nearly parallel north and south ranges, from 30 to 60 miles apart, and separated by a tract of lower but hilly country with a generally southern slope, where many streams unite to form the southward-flowing Connecticut River.

The Adirondack Mountains, in northeastern New York, are situated some 25 miles to the west of the Green Mountains, and separated from them by another tract of hilly country similar to the one dividing the mountains of Vermont from those of New Hampshire. In this space lies the irregular sheet of water over 100 miles long known as Lake Champlain. This beautiful lake discharges northward through Richelieu River to the St. Lawrence. In the same tract of hills, but to the southward of Lake Champlain and tributary to it, lies the smaller but still more charming Lake George.

The Adirondacks are rudely circular in ground plan, and measure from 60 to 70 miles from east to west, and about 100 miles from north to south. The entire area, known to the early settlers of New York State as the North Woods, is rugged and most pleasingly diversified. Its leading charms are the large number of dark, densely forested summits, the many beautiful lakes and clear, sparkling streams. The highest of the numerous steep-sided peaks is Mount Marcy, 5,344 feet, and second in rank is the equally beautiful eminence known as Whiteface, which rises 4,872 feet above the sea and about 3,000 feet above the adjacent valleys. Over 20 neighbouring forest-covered summits have elevations in excess of 4,000 feet.

The rugged region in northeastern New York and the adjacent portion of New England is in general without well-marked boundaries. On the north it extends into Canada, and is margined by the great valley through which flows the St. Lawrence. In the province of Ottawa, to the south of the St. Lawrence, there is a group of bold hills similar in many ways to the Green Mountains, known as the Notre Dame Mountains, which decreases in height when traced northward and merge with a roughened plateau which extends far to the northeast and embraces the Gaspé Peninsula and the table-land and hills of New Brunswick. Much of the country adjacent to the St. Lawrence on the south is rolling and hilly and contains large tracts of rich agricultural land which is highly favourable for dairying and sheep-raising. Mount Sutton, the highest elevation in the Notre Dame Mountains, is 4,000 feet high, and several other forest-covered mountain-like hills range in elevation from 1,000 to 3,000 feet. In the irregular valleys of this region there are a large number of lakes, situated in general from 700 to 1,000 feet above the sea. The Gaspé Peninsula to the north of New Brunswick, bordered on the north by the valley of the St. Lawrence, and on the east by the Gulf of St. Lawrence, has a rough relief and dense forests and is still a wilderness. The general elevation of the uplands in this little known region is about 1,500 feet. The surface is in reality a broad plateau in which numerous valleys have been excavated and from which rises a range of hills termed the Shikshock Mountains, some 65 miles long and 4 or 5 miles wide, with peaks ranging from 3,000 to 4,000 feet in height.

Much of Maine and New Brunswick is similar to the region just referred to, and, in a generalized geographical view, may be considered as a part of the great coastal plateau of the northeastern portion of the continent, roughened by erosion so as to appear to one travelling through its valleys as an endless succession of rugged hills. The highest of the numerous prominences in Maine is Mount Katahdin, 5,200 feet, and in New Brunswick the culminating summit is Bald Mountain, 2,470 feet.

The rugged region embracing the Adirondacks, together with the more elevated portions of New England and of the adjacent provinces of Canada, has many geographical features that are similar to those of the southern Appalachians, but at the same time this, the central portion, differs in a marked way from the southern extension of the Atlantic mountains. The higher mountains in each of these picturesque regions are at least in a general way to be considered as the unconsumed remnants of ancient uplands, the greater part of which have been eroded away. The most marked contrast in the scenery of these two regions of similar elevation is due to the presence of a great number of lakes at the north, many of them of large size, and the total absence of such beautifying elements in the landscapes at the south. The streams at the north are frequently impetuous and broken by many cataracts and rapids, thus furnishing abundant water-power; while at the south the streams flow through more evenly graded channels and are without cascades except near their sources in the mountains. These contrasts are such as are to be found the world over between regions of young and old topography. The differences in the degree of development reached by the streams of the New England region as contrasted with those of the southern Appalachians, finds an explanation in the fact that New England, Canada, etc., was formerly covered with glacial ice, and on the retreat of the glaciers the surface of the land was left with an essentially new relief, while the southern Appalachians were well to the south of the great ice invasion, and the streams of that region have reached a mature development, except near the sources of their head-water branches, which, like the topmost twigs of a tree, are always young.

The central, like the southern portion of the Atlantic mountains, is forest-clothed. All but a few of the highest summits in the Adirondack and White Mountains are concealed beneath a dense and varied growth of trees and shrubs. The summits, which are nearly bare of vegetation, like the upper 800 or 1,000 feet of Mount Washington, owe this condition to lack of soil rather than to elevation. Nowhere in the Atlantic mountains to the south of the but little known peaks near Hudson Strait, is the elevation sufficient to reach above what would be the timber-line under favourable soil conditions. The trees of the White and neighbouring mountains are principally various species of conifers, such as the pine, spruce, hemlock, larch, etc., which grow thickly on all but the most precipitous slopes. Before man disfigured the beauties of the land the lower hills, the river-valleys, and the borders of the numerous lakes and tarns were clothed with a more varied flora than the uplands. In these valley forests the dark foliage of evergreens is in summer mingled with the lighter green of maples, beeches, birches, oaks, locusts, and other broad-leaf trees. The forests are thus highly diversified and partake of the characteristics of both the northern and southern floras. It is in these northern woods that the glorious autumnal colouring for which North America is justly famous is to be seen in its greatest splendour. October is here truly the golden month of the year. At that season the bold hills, with their sombre robes of coniferous trees, rise like dark rugged islands above an undulating sea from which the most gorgeous sunset colours seem to be reflected. The brilliant colouring of the ripe foliage beautifies the land as with a cloth of gold. It is at this season also, during the tranquil days of what is known as Indian Summer, that a purple haze is thrown like a veil over the harlequin landscape, as if to subdue its glories and bring them within the range of man's appreciation.

Only a few of the higher summits in the New England region approach the scenic conditions usually associated with truly lofty mountains. In fact, the general lack of rugged escarpments as well as of great elevation leads the geographer to rank even the highest of these rounded summits as hills of large size rather than attempt to burden them with the dignity that the term mountain carries with it. They are beautiful hills, separated one from another by lovely valleys, which draw the beholder to them and fill his memory with tender longings and vague dreamy fancies such as the sterner grandeurs of great mountains fail to awaken.

The Laurentian Highlands.—A vast area in the eastern portion of Canada, to the north of the valley of the St. Lawrence, including Labrador, is underlaid by very ancient crystalline rocks of the same general character as those forming the Adirondacks. This same geological system, the Archean, has a wide development in the continental basin to the north of Lake Superior and about Hudson Bay. To the north of Quebec, in the region drained by the Saguenay and Ottawa Rivers, the land has a general elevation of 1,500 to 1,600 feet, and is known as the Laurentian Highlands, although sometimes dignified by the name Laurentian Mountains, or, more briefly, as the Laurentides. In reality, this broad, indefinitely defined region from a geographical point of view is a roughened plateau and not a mountain range or group of ranges. When the structure and metamorphosed condition of the rocks are considered, however, it is found that they have the characteristics pertaining to the central and more deeply seated portions of true mountains. The rocks are mainly crystalline schist, gneiss, granite, etc., together with igneous intrusions, all of which have been intensely folded, crumpled, and broken. The general interpretation of the existing conditions is that deep erosion has occurred and, in fact, a mountain range or a mountain chain worn down to a generally plane surface. The thickness of the rocks thus removed, or the depth of erosion, is unknown, and owing mainly to the complexity of the geological structure of the terranes remaining, will perhaps never be ascertained, but can be safely estimated as not only hundreds, but several thousands of feet. Erosion has laid bare portions of the earth's crust which were once deeply buried, and reveals the character of the "basement complex," as it has been termed, which forms the foundation of the continent. Owing to the great age of the rocks and the depth to which they were once depressed in the earth's crust, they have experienced great changes. They are not only intensely folded and crushed, but in large part have been caused to flow under great pressure, and have thus acquired a schistose structure. Fissures have been filled with molten rock injected from below so as to form dikes, and possibly still greater or regional intrusions have occurred. Over large areas the amount of once molten and intruded rock exceeds the surface exposure of what are usually, but with some hesitation, classed as metamorphosed sediments.

Long exposure to the air in a region of mild relief is usually accompanied by the formation of a deep soil. The soil over the Laurentian Highlands, however, is generally thin, and large areas of bare rock are exposed. The explanation of this apparent anomaly is that glaciers during a geologically recent period were formed on this region and flowed away from it, carrying most of the previously formed rock débris with them. The time since the melting of the glaciers has been too short for a new soil to form, except in the valleys and depressions among the bare glaciated hills, which hold a peaty accumulation resulting from the partial decay of vegetation. The scarcity of soil is also due in part to the climatic conditions now prevailing, which are unfavourable to rapid rock decay.

To the north of the Laurentian Highlands and in the vicinity of Hudson Strait, the land becomes higher, and as recently reported by Robert Bell, of the Canadian Geological Survey, forms true mountains with elevations in the neighbourhood of 8,000 feet. What revelations are to come from the inhospitable and in large part ice-covered lands still farther north can only be told as exploration and surveys are extended in that direction.

This brief review of some of the leading characteristics of the mountains and hills adjacent to the Atlantic coast will, I think, serve to show that they bear a family relationship; like the members of a family, they are of various ages, although all of them are past their prime, and may with propriety be termed the Atlantic Cordillera.

An inspection of the map forming Fig. 14, on which the larger geographical features of North America are indicated, will assist the reader in appreciating the general relations and extent of the plains and plateaus which collectively form the Continental basin.

This medial region of the continent is bordered on the east for some 2,000 miles by the Atlantic mountains, and on the west throughout its entire extent by the Pacific mountains. It is open to the sea at both the north and the south, and extends in one continuous series of plains and plateaus from the Gulf of Mexico to the Arctic Ocean. The southern portion of this interior basin or trough has already been briefly described in discussing the characteristics of the Gulf plains. The northern portion has also been considered in describing the tundra region adjacent to the Arctic Ocean.

The leading geographical features of the North American continental basin are its generally low elevation, the mildness of its topographic details, and, with two exceptions, the absence within its borders of elevations having a mountainous structure. In general the rocks beneath the surface are horizontally stratified marine sediments. The stream-cut valleys are shallow and usually broad, except in the bordering plateaus and foot-hills on the east and west sides, where the streams frequently flow several hundred feet below the surface of the broad, flat-topped interstream spaces. The drainage of the continental basin serves as a convenient basis for subdividing it into three separate portions. These are the Gulf slope, which discharges its surplus waters into the Gulf of Mexico and is drained principally by the Mississippi; the St. Lawrence slope, occupied in part by the Great Lakes and drained by the St. Lawrence River; and the arctic slope, down which the Mackenzie, Nelson, and other rivers flow to the Arctic Ocean or to Hudson Bay. At no place are the Pacific mountains broken by cross-drainage, so as to allow the continental basin to send a tribute to the Pacific Ocean.

The vast extent of the Continental basin, embracing, as it does, some three-fourths of the entire area of North America, makes it necessary, even in a general review of the large geographical features of the continent, to recognise smaller subdivisions than the three great drainage slopes referred to above. For this purpose we select the more or less well-defined plains and plateaus into which the region is naturally subdivided. The portion of the Continental basin embraced within the boundaries of the United States has been shown by J. W. Powell to consist of the following physiographic regions, namely, the Gulf plains; the Prairie plains; the Lake plains, including the region draining to the Great Lakes; and the Great plateaus or Great plains, as they are more generally termed, adjacent to the eastern border of the Pacific mountains. Several of these divisions need to be extended and still others recognised in order to include the entire region under review. The portion of the Continental basin to the north of the United States-Canadian boundary has been only partially explored, and the subdivisions of it suggested below are to be considered as provisional.

The Lake plains include in Canada the country to the north of the Great Lakes, which drains to them, but excepting the flat lands bordering Lakes Erie and Ontario and once covered by their waters, the region referred to is rather a roughened plateau than a plain. From a geological point of view the hilly country composed of crystalline rocks to the north of Lakes Superior and Huron and included within their hydrographic basins partakes more of the character of the Laurentian Highlands than it does of the features of the portion of the Lake plains situated in the United States.

The Prairie plains also extend far to the north of the international boundary, and on their northern border merge with the forest-covered plains in central Manitoba and the northern portion of Saskatchewan, which are drained by northward-flowing rivers. These plains in the far north differ from the Prairie plains in the fact that they are forested and acquire greater diversity from the presence of innumerable lakes, several of which are of large size. For convenience we may designate this vast and but little known northern region as the Subarctic Forest plains. Still farther north, where the forest dies away, lie the Barren Grounds, which merge on their northern border with the frozen morasses or tundra of the arctic coastal plain.

To acquire just conceptions of the topographic and other characteristics of the several regions of mild relief which make up the Continental basin is a difficult task, as each one is of great extent and possesses many peculiarities of its own, and besides, in two separate regions, each embracing many hundreds of square miles, movements in the earth's crust have occurred of such a nature as to elevate the rocks and give them the general structure commonly found in mountain ranges. Reference is here made to the Ozark uplift in the southwestern portion of the Prairie plains and the Black Hills of Dakota which rise from the Great plateaus.

The Ozark Uplift.—There is an area embracing about 75,000 square miles in southern Missouri, northern Arkansas, and the eastern border of the Indian Territory, in which the rocks have been upraised above the surrounding Gulf and Prairie plains. The uplift, if we imagine it uneroded, would have the general form of an inverted canoe; that is, it would form an elongated ridge, broad and dome-like in the central portion and dying away on all sides into the great surrounding region of undisturbed and essentially horizontal rocks. The major axis of the uplift, although exhibiting a double curvature, has a general northeast and southwest trend. It is about 500 miles long, and in the widest part is approximately 200 miles broad. What the height of the dome would be had the rocks composing it not yielded to the destructive influences of the air or been removed by streams cannot be readily estimated, since the movements of the earth's crust which upraised it occurred at several widely separated intervals with intervening periods of decay and erosion, and downward movements have also been experienced which submerged the region and permitted the deposit of sheets of sediment over it. If the results of the upbuilding agencies had not in a large measure been counteracted in these several ways, the dome to-day would have a height of several thousand feet. In the present condition the deeply eroded dome presents the net result of elevation over subsidence and erosion. The dome-like form is lost, and in its place is a complex series of ridges and valleys. The higher summits now remaining, situated principally in the Iron Mountain district in northeastern Missouri, rise from 1,400 to 1,800 feet above the neighbouring plains, and from 1,800 to 2,100 feet above the sea.

The greater intensity with which the rocks in the southwestern portion of the Ozark uplift have been folded than in the more northern portion and the varying degrees to which the beds have yielded to denudation have resulted in giving to its various parts different types of topography. This diversity has led to the recognition of several distinct divisions, such as the Shawnee Hills, at the extreme northeastern end of the uplift, where the rocks have been folded and the ridges cut across by the Mississippi; the St. François Mountains, in southeastern Missouri, composed of a large number of isolated hills and rising from 500 to 800 feet above the adjacent valleys; the Ozark plateau, in southwestern Missouri and northwestern Arkansas, the central part of which has a general elevation of 1,500 feet above the sea, and to one travelling over it seems a boundless and featureless plain underlaid by apparently horizontal but in reality gently westward dipping sheets of stratified rocks; the Boston Mountains, in central and western Arkansas, consisting of rugged irregular ridges and truncated summits with a general crest-line elevation of 1,000 feet above the sea; and the Ouachita (pronounced Wichitaw) Mountains, formed of numerous rudely parallel upward folds of hard rock, which rise from 500 to 1,000 feet above the adjacent valleys and form a belt of unusually picturesque, forest-crowned hills, extending from Little Rock, Arkansas, westward into Indian Territory.

The study of the island-like Ozark region in the broad, ocean-like expanse of the prairies is far from being complete. Although topographically distinct and appearing as one of the minor units in the geology of the continent, geologists are inclined to the view that the Ozark uplift as above described should be considered as consisting of two independent but contiguous areas of upheaval, namely, the Ozark Hills, situated mainly in Missouri, and the Ouachita Hills, lying mainly in Arkansas and the Indian Territory. This Ozark-Ouachita region—by whatever name finally designated—is one with a long and varied, nay, even a poetic history. In writing of the Archean rocks of the Iron Mountain region, Missouri, Arthur Winslow states that they "are truly ancient elevations, older than any others in the State, older than the mountains of Arkansas, older than the Appalachians, older than the Rocky Mountains; if venerable be an attribute of great age, they certainly merit that appellation. For not only are all other rocks of Missouri youthful as compared with these, but there is a genetic relationship, and the former are in a sense descendants of the latter. For when the limestones and other sedimentary rocks were yet unformed these crystalline rocks must have existed as parts of a continental mass, and from the degradation of this continent resulted the materials of the later formed sedimentary rocks. The present granite and porphyry hills are but protruding parts of the remnant of this ancient continent which stood as islands above the ocean waters while the beds of limestone and sandstone were being formed about them, which rose with these beds when they were lifted from the waters, which now, rugged and weather-beaten, yet tempered by age and varied experience, rear themselves above the surrounding younger rocks and bid fair still to live when the latter have yielded to the forces of degradation."

Besides its pleasing scenery, varied and abundant mineral resources, and health-giving springs, this oasis of hills amid the unvaried monotony of the grass-covered plains in the southern portion of the continental basin derives an additional attraction from its forest growths in which southern pines are mingled with oaks, hickories, walnuts, and other broad-leaved trees. The soil is generally productive, and great fields of corn and cotton may be seen side by side.

The Gulf Plains.—The Gulf plains include the western portion of Florida, and extend westward and southward about the borders of the Gulf of Mexico in a continuous belt from 50 to 60 to perhaps 100 miles wide, to where the Pacific mountains approach the coast in east-central Mexico. This low, gently seaward-sloping region, underlaid by soft horizontal strata, possesses a generally rich soil well adapted for the cultivation of cotton, corn, sugar-cane, and rice. In the low, hot country of eastern Mexico nearly all tropical fruits can be successfully raised. The most characteristic as well as the broadest portion of this productive belt is in the States of Mississippi and Louisiana, and extends northward with a gradually decreasing width to the mouth of the Ohio. This is the lower Mississippi basin, which owes its existence mainly to the deposits of silt laid down by the river after which it is named. Much of the land is really the delta of the "Father of Waters," over which that river spreads out in vast inundations each year.

The Gulf plains skirt the southern end of the Appalachian Mountains, and to the west of the Mississippi are bordered in part by the Ozark uplift. There are certain reasons for believing that these two regions of elevation, characterized by a similar geological structure, are portions of a single greatly disturbed belt, but are now separated by a broad area which has been depressed and deeply covered with comparatively recent sediments. But that this general view of the origin of the larger features in the relief of the Gulf States can be accepted with entire confidence is questionable. True it is, however, that the delta region of the Mississippi has undergone many up and down movements, and that several successive sheets of sediment have been laid down upon it, but that the folds and crumplings characteristic of the southern Appalachians and of the Ozark uplift extend across the intervening space beneath the covering of horizontal rocks has not been demonstrated.

The Gulf plains throughout are less than 500 feet above the sea, and much of the Gulf margin and the similar tract which extends northward to the mouth of the Ohio has an elevation of less than 100 feet. The fringe of lowland bordering the Gulf and extending up the course of the Mississippi is generally swampy and contains numerous small water bodies which owe their existence to the cutting off of the beds of the river so as to form what are termed ox-bow lakes.

Previous to the settlement of the Gulf plains by Europeans and the clearing of much of the land for plantations it was clothed with such a dense growth of trees and vines as to be almost impenetrable. The southern pine there reaches its greatest perfection and is the basis of a great lumber industry, and oaks of several species, the wide-spreading white-trunked sycamore, the still more stately tulip-tree with its cup-like blossoms of yellow, the fragrant magnolia, the seemingly always aged cypress, the gum-tree, and many other species of arboreal vegetation also find most congenial conditions for their growth. The dwarf palmetto, which forms such a characteristic growth in Florida, extends northward in the Mississippi basin to the southern border of the Ozark uplift. Much of the luxuriant moss and lichen draped forest of the Gulf plains with all its primitive network of shrubs and vines still remains.

The Prairie Plains.—A prairie in the current use of the term is a generally level region, either a plain or a plateau, without forests but clothed with a carpet of luxuriant grasses and flowering annuals. A rolling prairie is an undulating or hilly, grass-covered region. The Great plains of the west-central portion of the Continental basin meet these requirements, and are typical prairies. On their eastern and northern border the Prairie plains merge with the adjacent forested plains, and on the west from Mexico northward to the subarctic forest pass by still less tangible gradations into the more elevated and drier Great plateaus or high plains, where bunch-grass, with bare intervals between the scattered tufts, takes the place of the continuous sod of the true prairies. The reasons for the change from forest to prairie and beyond to the land of the bunch-grass as one travels from east to west across the interior basin, lie in differences in the humidity of the climate.

The Prairie plains have their beginning at the south in Mexico a short distance from the Rio Grande, and are prolonged northward through central Texas, meeting to the north of Red River the forest-covered Ouachita Hills. But to the west of the Ozark uplift the Prairie plains extend northward in a belt about 100 miles wide which expands in Kansas, northern Missouri, eastern Nebraska, Iowa, Illinois, and western Ohio to fully 800 miles. In this highly fertile region, now the most productive agricultural area of comparable size in North America, if not in the world, one may travel in a straight line for nearly 1,000 miles through a land without high hills but pleasingly diversified by undulations of the generally level surface and by winding stream-formed valleys bordered by swelling bluffs, without losing sight of towns, villages, or comfortable farmhouses. In spring this entire region is bright green with pastures and sprouting grain-fields, and in autumn yellow with the harvest. Miles on miles of rustling corn-fields form the most characteristic feature of the summer landscapes.

The Prairie plains contract to the north of Illinois and Iowa to a width of about 200 miles, being encroached upon by the forests of the Great Lakes region, but are prolonged northward through Minnesota and the Dakotas far into Canada. The length of these natural meadows from south to north is nearly 2,000 miles; their entire area is not far from 500,000 square miles. On the north they merge with the vast region of similar relief which is darkened by the pines and spruces of the subarctic forest.

The northern portion of the original prairie region has been given a new and in some respects a more pleasing aspect by the sowing of millions of acres with wheat. This is the most favourable large area for wheat culture in North America, and one of the three great wheat-growing regions in the world. The most productive portion of these northern wheat-lands lies in the valley of the Red River of the North, situated in part in Minnesota and the eastern portion of the Dakotas, but including also the plains of Manitoba. Could we view the broad extent of the Prairie plains as do the birds in their southward migrations, we would see them golden with the sheen of ripening wheat at the north, green and russet in the central portion with corn, and white with cotton to the south. Everywhere from south to north and east to west the vast expanse is dotted with the curling wreaths arising from household fires, and at hundreds of localities blotted by the smoke of towns, factories, smelting-works, and coal-mines.

Throughout the entire extent of the Prairie plains the underlying rocks are essentially horizontal, and consist largely of limestone. An ancient sea-bottom has been broadly upraised with but slight disturbances of the strata to a general elevation of about 800 feet in Minnesota and the Dakotas. From this low continental divide the land slopes gently both to the north and south. The local variations of surface are due mainly to the unequal weathering of the rocks and the excavation of stream-formed valleys. To the north of the mouth of the Ohio, however, the prairie, in common with the adjacent regions, was formerly occupied by glacial ice, which on melting left widely spread deposits of clay, stones, gravel, etc., which gave the region a new surface, and in certain instances turned the streams from their former courses. Much of the rolling prairie inherits its billowy surface from the glaciers. In the midst of the young topography of glacial and more recent date there is an area of about 10,000 square miles in southwestern Wisconsin and adjacent portions of Minnesota and Iowa which is surrounded by the deposits of the ancient ice-sheets (glacial drifts), but not covered by them. This driftless area, as it is termed, has an old topography in striking contrast to the relief of the region about it, in which broad river-valleys bordered by the pinnacled and castellated rocks exposed in the bordering slopes of the adjacent uplands are among the most conspicuous features.

The soil of this driftless region is a ferruginous clay, resulting from the prolonged weathering of the rocks, principally limestone, on which it rests, while the surfaces formerly covered by glacial ice are mantled with soil of a mixed character containing many fragments and large boulders of compact rock. In the prairies to the south of the glacial boundary the soils are mainly of a sedentary origin, and have resulted from the disintegration and decay of the rocks on which they rest, but usually rendered black by the humus resulting from the partial decay of numberless generating grasses and other lowly plants. This black soil is wonderfully productive and furnishes the basis of the greater part of the wealth and industries of the region it covers. The minor exceptions to the general fertility occur where the rocks immediately underlying the surface, as in the zinc and lead region of southwestern Missouri, are highly charged with flint-like material, which remains when the limestone once containing it is dissolved and carried away. The horizontal sheets of rock beneath the broad central portion of the Prairie plains belong to the Carboniferous system and contain highly valuable seams of bituminous coal. The area of these coal-producing lands is estimated at 125,000 square miles. In this same region also there are extensive tracts in which natural gas and petroleum are obtained in remarkable abundance. In southern Wisconsin and the adjacent portions of Illinois valuable deposits of lead occur under conditions similar to those associated with the lead and zinc mines about the northern border of the Ozark uplift.

Owing to the demand for transportation facilities and the mild relief of the land, the entire extent of the Prairie plains is covered with a double-lined network of steel. The ganglia in this pulsating nerve system of intercommunication are Chicago (here included, as it belongs to the prairie as well as to the Great Lakes region), St. Louis, Kansas City, Omaha, Minneapolis, St. Paul, etc., cities with from 100,000 to over 1,500,000 inhabitants, and hundreds of lesser centres of trade, manufacture, and education.

The Lake Plains.—The region draining to the Great Lakes—or the Laurentian lakes, as they may, perhaps, be more properly designated, since they form the source of the river of that name—presents many striking contrasts to the more monotonous treeless prairies skirting it on the south and west.

The name "Lake plains," suggested by J. W. Powell for the portion of the region here referred to within the borders of the United States, when extended to the entire area draining to the Laurentian lakes, is in part a misnomer, since much of its surface is rough and irregular. In a certain sense, however, the term plain is applicable, since it includes a plain of water over 95,000 square miles in area. The combined areas of the lakes are greater than that of the region draining to them. The land bordering the Laurentian lakes is underlaid to a large extent by horizontal or but slightly disturbed sedimentary rocks, but includes on the north a portion of the contorted, crystalline terranes already referred to as forming the Laurentian Highlands, and in general is characterized by the mildness of its relief. The elevations of the surfaces of the several Laurentian lakes above the sea are, in feet, as follows: Superior, 602; Michigan and Huron, 582; Erie, 373; and Ontario, 247. The land forming the margins of these water bodies rises in general less than 300 feet above their surfaces. In portions of northern Michigan and in the region of crystalline rocks to the north of Lakes Superior and Huron, however, the relief is more pronounced and there are many bold rounded hills with basins between them.

The principal part of the nearly plane land surface about the Laurentian lakes is in immediate proximity to their borders, and records the former extent of their waters. These plains, composed of clay deposited from the lakes when more widely expanded than at present, form a fringe from 5 to 50 or more miles broad all about the present lake margins. Across this gently sloping surface the streams from the uplands, increasing in length as the lakes were lowered, have excavated narrow, steep-sided channels. These modern plains furnish typical illustrations of young topography.

In its primitive condition nearly the entire Laurentian lakes region was densely covered with trees. Previous to the destruction which followed the advance of the lumbermen its northern portion contained some of the finest and most valuable white-pine forests on the continent. To the south of the Laurentian Lakes, and in a general way adjacent to the Prairie plains, there were park-like areas in the forest, known as oak-openings, where picturesque bur-oak grew in open groves amid luxuriant natural meadows. These sunlit gardens, yellow and purple with golden-rods and asters in autumn, owed their existence to soil conditions determined long previously by the streams issuing from the margin of the retreating ice-sheet, which formed level areas of sand and gravel. The loose open texture of these deposits renders them less retentive of moisture than the neighbouring morainal hills, and during the long hot summers all but the most deeply rooted of the trees that spring up upon them perished.

The soil throughout the Great Lake region is nearly all of glacial origin and presents many local variations, dependent principally on the fact that the streams flowing from the ice assorted the débris delivered to them. The surface material, technically speaking, is of both glacial and fluvio-glacial origin. The former consists principally of stony clay or till, and the latter of gravel. About the immediate border of the existing lakes lacustral clays form the surface. The leading characteristics of the glacial and fluvio-glacial soils are their varied composition and endurance under cultivation. The glaciers that ploughed the land preparatory to the present harvest gathered together a great variety of rock débris, much of it broken and unweathered and not leached of its more soluble constituents.

The most typical portion of the Lake plains, including the southern part of the province of Ontario and the southern shores of the Laurentian lakes from Minnesota to New York, is highly favourable for agricultural pursuits, and produces in abundance a great variety of crops as well as richly flavoured fruits, luscious berries, and healthful vegetables. The beneficial influence of the neighbouring water bodies on the climate, tempering the heat of the summers and moderating the severity of the winters, is shown especially in the distribution of the fruit belts of Michigan, Ohio, and New York, which are in regions where the prevailing winds blowing over them come from the lakes.

The Subarctic Forest Plains.—The Prairie plains merge at the north with a great tract of forest-covered lowlands, which extend from the Laurentian hills on the east to near the base of the Rocky Mountains on the west. The change as one travels northward from the grassy prairies to the country of equally mild relief, but clothed with trees adapted to a rigorous climate, is gradual. Along the irregular and in part indefinite junction of these two vast plains, the alignment of the forest is broken in many places, and its margin fringed by a picket-line of groves and of isolated trees, which has advanced southward and invaded the grass-lands. Between these outposts the prairie with its wealth of summer bloom reaches well into the realm of perennial shadow. The southward extensions of the forest are mainly in the valleys and adjacent to the streams, while the drier steppes between are open grass-lands. No conspicuous change in the topography of the land or of the rocks or the soil coincides with the change from grass to forest. The differences in vegetation must therefore be sought in climatic conditions, and mainly in the influence of atmospheric changes on the water contained in the soil.

Throughout practically the whole of the region occupied by the subarctic forest, between Hudson Bay and the foot-hills of the Rocky Mountains, the land is low and the valleys monotonous. Many lakes are present, several of them of large size, and the rivers are remarkable for their lengths, low gradients, and large volumes.

The subdued topography of the region here considered, and the presence of vast numbers of lakes and swamps, is due in general to the influence of the ice-sheets which formerly covered it. In a minor way the presence of the innumerable small lakes and swamps is owing to the obstructions formed by growing vegetation, the damming of streams by driftwood, the work of beavers, and possibly the influence of subsoil ice.

To the north of the Subarctic Forest plain, as already described, occur the desolate tracts known in Canada as the Barren Grounds, which form a part or merge into the tundras bordering the Arctic Ocean.

The Great Plateaus.—The boundary between the prairie plains of the central portion of the interior Continental basin and the Great plateaus (Great plains) bordering them on the west is usually indefinite. The prairies pass into the more elevated and drier plateaus by insensible gradations. The plateaus rise gradually from east to west, and along their western margin, adjacent to the east base of the Pacific mountains, attain a general elevation of from 5,000 to 6,000 feet. Over vast areas these monotonous plateaus, with their even sky-lines, are higher above the sea than the crests of the Appalachians, and along their western margin in many localities even surpass in elevation the most prominent peaks in the eastern portion of the United States. Accompanying this increase in elevation from east to west there is a decrease in precipitation, and in consequence a marked change in the vegetation. The plateaus, like the prairies, are treeless in their most characteristic portions, but the larger rivers winding across them are margined in many instances by giant cottonwoods.

The mental picture that a traveller over the broad plateaus retains in after-years is of a vast treeless level tract of country, boundless as the ocean, which is bright green and decked with lowly flowers in the early spring, but becomes yellowish brown as the heat and dryness of summer increase and the grasses lose their freshness. Various portions of the plateaus, however, have their own individuality and present characteristics which make them conspicuously different from other portions of the same great series of steppes. At the south, in the region of the Rio Grande and of the Pecos and Canadian Rivers, the plateau is dissected by stream-cut valleys 1,000 feet or more deep, and from one to two score miles across, which divide it into a number of individual table-lands. The plateau margins for many miles on each side of the larger river-valleys have been carved by a complex system of secondary and usually ephemeral streams into a great variety of rock forms with deep trenches between. These conspicuously sculptured areas constitute what are commonly termed Bad Lands. In certain regions also the surfaces of the plateaus, more especially in Nebraska and South Dakota, are broadly undulating or reveal a seemingly endless succession of ridges and hills separated by shallow depressions, due to the presence of large tracts of drifting sand. In spite of these several variations, however, the leading characteristics of by far the larger portion of the plateau country are the generally level grass-covered surfaces extending away in all directions far beyond the reach of vision. On the rolling prairie one can frequently see the undulating surface about him for a distance of 15 or 20 miles, but the curvature of the earth usually draws still narrower limits to the region within the view of the plainsman. In riding over the plains the scene changes but little from day to day and from week to week. Monotony is the one word that best describes the lives of those whose lot is cast on these broad featureless surfaces. In journeying westward across the plateau over any one of the transcontinental railways a moment of excitement occurs when the even line of the western horizon is broken by the summit of a cloud-like mountain-peak. "Land ho!" is no more thrilling to voyagers on the ocean than the shouts which first made known the presence of a mountain-peak to the bands of immigrants who slowly voyaged across this sea of grass with their picturesque "prairie schooners" previous to the building of the railroads which now bind together the East and the West.

The Great plateaus begin indefinitely to the south of the Rio Grande, broaden in the United States to a general width of about 400 miles, and extend far northward into Canada. Their northern limit has not as yet been determined, but is to be looked for near the head waters of the Mackenzie. The length of the plateau country is in the neighbourhood of 2,000 or 2,500 miles, and its average width about 300 miles. An estimate of the area with a generally plane surface and an elevation of from 1,000 to 6,000 feet above the sea places it at about 700,000 square miles.

The eastern portion of the Great plateaus includes western Texas, Oklahoma, the central and western portions of Kansas and Nebraska, the western half of South Dakota, western North Dakota, western Assiniboia, and thence extend northward so as to include portions of Saskatchewan, Alberta, and Athabasca. On the west the plateau region includes the eastern portions of New Mexico and Colorado, extends far into Wyoming, and embraces central and eastern Montana, and thence reaches northward to Mackenzie.

This region of essentially level plateaus, extending as it does from the hot lands of eastern Mexico nearly to the arctic circle, presents great diversity of climate and also well-marked variations in the secondary features of its relief. Of necessity it needs to be subdivided for more detailed study. The rivers flowing eastward from the Rocky Mountains have excavated valleys in the plateau region, and may be used as a basis for its subdivision. This has been done by J. W. Powell for the portion within the borders of the United States, and the terms Pecos plateau, Arkansas plateau, Platte plateau, and Missouri plateau have been proposed; this category may be extended especially to the northward, so as to include the less well-known Saskatchewan, Athabasca, Peace, and Laird plateaus. Each of these divisions is in reality a group of plateaus, for the reason that the broad areas between the eastward-flowing rivers are trenched by lateral stream channels tributary to the main waterways, and thus subdivided into smaller units. This subdivision of the plateau region by stream channels leaving flat-topped areas between them makes one instructive geographical process prominent—that is, the great table-land has been dissected. The depths of the channels cut across it depend mainly on the elevation of the land and the distance the streams have to travel to reach the sea; but modifying conditions are furnished by the degree of resistance the rocks offer to erosion, the amount of precipitation, etc. If the elevation is great, the stream can cut deeply, and leave bold secondary plateaus between them; if the distance to the sea is short, other conditions being the same, the streams can cut more deeply than when their courses are long; if the rocks are resistant, they are left in bold escarpment bordering the valleys and the margins of the secondary plateaus are well defined, but if they are soft and crumble easily, their débris is washed and blown into the rivers, and a general lowering of the surface without the formation of deep trenches is the result. These and still other conditions have influenced the manner in which the Great plateaus have been dissected, and are of necessity to be considered in a critical discussion of the history of the land as recorded in its relief.

The main reason for the dissection of the region under consideration is to be found in the fact that it is bordered on the west by high mountains where precipitation is abundant, and the streams, supplied largely by the melting of the snow in summer, flow across a comparatively rainless country. The stream channels in general have been deepened at a more rapid rate than the areas between them have been lowered by erosion. Valleys running east and west have thus been excavated, leaving the intervening spaces as uplands, which, however, in certain instances have been minutely dissected by the streams originating on them and supplied by local winter precipitation. Added to these general conditions are differences in rock texture, which have led to great variations in the details due to erosions, particularly on the valley borders.

One other condition which has modified the history of the plateau region throughout, but most decidedly at the north, is the climatic change which culminated in the Glacial epoch. During the time referred to the northern portion of the Great plateaus situated in Canada and the adjacent part of the United States was invaded by glacial ice which spread an irregular sheet of detritus over the country it occupied. Decided changes occurred also in the central and southern portion owing to increased precipitation, the flooding of the rivers leading from the melting ice-front, and to movements in the earth's crust of as yet undetermined extent and amplitude. It is apparent to the geographer that much of the history of the climatic changes of glacial and post-glacial times is recorded in the relief of the interior Continental basin to the south of the limit reached by the ice and in the terraces and alluvial deposits of the valleys, but as yet for the most part this interesting story remains unread.

The most deeply dissected portion of the Great plateaus occurs in western Texas, eastern New Mexico, and Oklahoma. In that region the rivers having their sources in the Rocky Mountains and flowing to the Gulf of Mexico have excavated deep and wide valleys, leaving broad intervening areas in bold relief.

The Pecos River drains a large part of the mountainous region in eastern New Mexico, and flows through a valley of its own making, which is some 30 or more miles broad and its bottom about 1,200 feet below the general surface of the plateau lying to the eastward. The Canadian River has excavated a similar valley, which is some 40 miles broad throughout much of its course, and is bordered by bold rocky escarpments from 1,000 to 1,200 feet high, in which the edges of the horizontal strata underlying the adjacent plateaus are exposed. This region of large and strongly defined topographic features illustrates in a remarkable manner the nature of the work performed by streams which rise amid high mountains and flow across a dry plateau standing well above sea-level.

El Llano Estacado.—A typical portion of the great plateau region left by deep dissection is furnished by the table-land named by early Mexican explorers "El Llano Estacado," or the Staked Plains, in reference to the fact that owing to the monotony of the surface and the scarcity of water the routes of travel were at first marked by stakes. This region, celebrated in the traditions of the Southwest frontier, is described by Captain Marcy, who crossed its eastern portion in 1849, as being "much elevated above the surrounding country, very smooth and level, and spreading out in every direction as far as the eye can penetrate, without a tree, shrub, or any other herbage to intercept the vision. The traveller in passing over it sees nothing but one dreary and monotonous plain of barren solitude. It is an ocean of desert prairie, where the voice of man is seldom heard, and where no living being permanently resides. The almost total absence of water causes all animals to shun it; even the Indians do not venture to cross it, except at two or three points, where they find a few small pools of water." As will be shown below, the barrenness and desolation of this arid tract is not so great as it seemed to those who first invaded its primeval solitude.

El Llano Estacado, or the Llano, as it is frequently termed, is about 500 miles across from north to south, and 280 miles wide from east to west. It is bordered on nearly all portions of its periphery by descending escarpments which lead down to the adjacent valleys. Its surface, although appearing horizontal, in reality slopes eastward at the rate of about 20 feet per mile, and on its highest, northwest border, has an elevation of 5,500 feet above the sea. This great table-land has a smooth floor, and, as reported by recent explorers, is clothed with an abundance of bunch-grass, which formerly furnished sustenance to herds of antelope and deer. It was in this general region also that some of the immense herds of buffalo which once inhabited the broad plateaus found a winter range.