SECTION II.—THE PLUTONIC ROCKS.

Granite is found to penetrate the stratified rocks in the form of veins. The following section (Fig. 3) will show the relation of granite veins to the granitic mass below. The granite which is quarried for architectural purposes is often in comparatively small quantities, disappearing at the distance of a few hundred yards beneath the stratified rock; or else it exists in the form of isolated dome-shaped masses. It is probable that, if they could be followed sufficiently far, they would be found to be portions of dikes coming from the general mass of granite below. Even the granite nuclei of the great mountain ranges may be considered as injected dikes of enormous magnitude.

Granite is itself intersected with granite veins more frequently, perhaps, than any other rocks; but the vein is a coarser granite than the rock which it divides. It is not uncommon to find one set of dikes intercepted and cut off by a second set, and the second by a third. The substance of the dikes was, of course, in a liquid state when it was injected, and the first must have become solid before the second was thrown in; hence the dikes are of different ages. The dikes a b c, represented in Fig. 4, must have been injected in the order in which they are lettered.

It is probable that, by the process of cooling, the liquid mass from which these dikes have proceeded has been gradually solidifying from the surface downwards. If so, it would follow that the granite nearest the surface (1, Fig. 2) is the oldest, and the newest is that which is at the greatest distance below (4). It is possible that at great depths granite may be still forming, that is, taking the solid form, though of this there can be no direct proof. There is, however, proof that it has been liquid at periods of time very distant from each other; for the dikes sometimes reach to the top of the coal formation (for example), and then spread themselves out horizontally, as at a, showing that the rock above the coal had not then been deposited. Another dike will extend through the new red sandstone, as at b, and spread itself out horizontally as before. These horizontal layers of granite, by their position in strata whose ages are known, indicate the periods when granite has existed in a liquid state. Granite veins have been discovered in the Pyrenees as recent as the close of the cretaceous period, and in the Andes they have been found among the tertiary rocks.

There are several other rocks, of minor importance, often found in connection with granite. Hypersthene rock, in a few cases, forms the principal part of mountain masses. Greenstone is more frequently associated with the trappean rocks, but it sometimes passes imperceptibly into syenite and common granite. Limestone is found in considerable abundance, and serpentine in small quantities, as primary rocks, and have evidently been formed like granite, by solidifying from a state of fusion.

SECTION III.—THE VOLCANIC ROCKS.

There are two principal varieties of lava, the trachytic, consisting mostly of felspar, and the basaltic, consisting of hornblende. When both kinds are products of the same eruption, the trachytic lava is thrown out first, and the basaltic last. The reason of this is, that felspar is lighter than hornblende, and probably rises to the surface of the lava mass at the volcanic focus, and the basaltic lava is therefore reserved till the trachytic has been thrown off.

These, like other rocks, have been produced at different epochs. There is, however, great difficulty in determining their age; There are some differences of structure and composition observed, in comparing the older and newer lavas; but the only method that can be relied on to determine their age is their relation to other rocks. When they occur between strata whose age is determined by imbedded fossils, they must be of intermediate age between the inferior and superior strata.

1. Modern Volcanic Rocks.—Some of the volcanic rocks are of modern origin, and are produced by volcanoes now active. The total amount of these, and of all the other volcanic rocks, is probably less than that of either of the other principal divisions of rocks; yet they form no inconsiderable part of the earth’s crust. The number of active volcanoes is not far from three hundred, and the number of eruptions annually is estimated at about twenty. In some cases, the lava consists of only a single stream, of but a few hundred yards in extent. It extends, however, not unfrequently twenty miles in length, and two or three hundred yards in breadth. The eruption of Mount Loa, on the island of Hawaii, in 1840, from the crater of Kilauea, covered an area of fifteen square miles to the depth of twelve feet; and another eruption of the same mountain, in 1843, covered an area of at least fifty square miles. The eruption in Iceland, in 1783, continued in almost incessant activity for a year, and sent off two streams in opposite directions, which reached a distance of fifty miles in one case, and of forty in the other, with a width varying from three to fifteen miles, and with an average depth of more than a hundred feet. The size of some of the volcanic mountains will also assist in forming an idea of the amount of volcanic rocks. Monte Nuovo, near Naples, which is a mile and a half in circumference and four hundred and forty feet high, was thrown up in a single day. Ætna, which is eleven thousand feet high, and eighty-seven miles in circumference at its base, has probably been produced wholly by its own eruptions. A large part of the chain of the Andes consists of volcanic rock, but the proportion we have not the means of estimating.

2. Tertiary Lavas.—There is another class of volcanic products, which are so situated with reference to the tertiary strata that they must be referred to that period. The principal localities of these lavas, so far as yet known, are Italy, Spain, Central France, Hungary, and Germany. They are also found in South America. Those of Central France have been studied with the most care. They occur in several groups, but they were the seats of volcanic activity during the same epoch, and formed parts of one extensive volcanic region. Each of these minor areas, embracing a circle of twenty or thirty miles in diameter, is covered with hills two or three thousand feet in height, which are composed entirely of volcanic products, like the cone of Ætna. On many of them there are perfectly-formed craters still remaining. Numerous streams of lava have flowed from these craters, some of which can now be traced, throughout their whole extent, with as much certainty as if they were eruptions of the present century. Some of the lavas have accumulated around the orifices of eruption, forming rounded, dome-shaped eminences. These lavas generally consist of trachyte, and have therefore a low specific gravity, and imperfect fluidity. The basaltic lavas have often spread out over broad areas, and, when they have been confined in valleys, have reached a distance of fifteen miles or more from their source. There still remain indications of a current of lava which was thirty miles long, six broad, and in a part of its course from four to six hundred feet deep. The above sketch (Fig. 5) will give some idea of the highly volcanic aspect which the district of Auvergne, in France, presents.

When we find that their activity commenced at so late a period and closed so long ago, we might be led to suppose that it was of very short duration. But a great number of facts, in the present condition of the country, require that we should assign to them a very prolonged activity. A single instance will be sufficient to show the nature of the evidence upon which this conclusion rests. The heavy line (Fig. 6) represents the present form of one of the valleys. A bed of lava forms the highest point of land represented, and a second bed is found in an intermediate part of the slope. The position of the upper bed must have been a valley, when the lava flowed there. We may represent this valley by the line a b c. The slow operation of natural denuding causes at length excavated the valley d e h, when another lava current flowed through it, covering its bed of pebbles, as before. The same denuding causes have at length produced the present valley, f g h. These remnants of lava-currents, as they have formed a very imperishable rock, have protected the subjacent strata from erosion, and furnish evidence of the position of the valley at different periods. When we consider with what extreme slowness denuding causes produce changes on the surface, and what extensive changes they have here nevertheless effected in the interval between the production of the different lava currents, we are compelled to feel that that interval was a very prolonged one. Yet this period, however long it may have been, was evidently less than the period of activity of these volcanoes.

The trappean rocks occur more or less abundantly in all countries. One of the most noted localities of this rock is a region embracing the north of Ireland, and several of the islands on the western coast of Scotland. It contains the celebrated Giant’s Causeway, which consists of a mass of columnar trap; also Fingal’s Cave, which is produced by a portion of the trap being columnar, and thus disintegrating more rapidly than the rest, by the action of the waves. An immense mass of greenstone trap, which has generally been considered as a vast dike, though often a mile in thickness, is found extending from New Haven to Northampton, on the west side of the Connecticut river. It then crosses to the east side, and continues in a northerly direction to the Massachusetts line. Under different names, it constitutes a nearly continuous and precipitous mountain range for about one hundred miles. Dr. Hitchcock supposes this greenstone range to be, not an injected dike, but a tabular mass of ancient lava, which was spread out on the bed of the ocean during the period of the deposition of the Connecticut river sandstone. It was subsequently covered with a deposit of strata of great thickness, and then by subterranean forces thrown into its present inclined position.

There is a mass of basaltic rock in the valley of the Columbia river, in the Oregon Territory, which extends without interruption for a distance of four hundred miles. Its breadth and thickness is not known, but in some places the river has cut a channel in this rock to a depth of four hundred feet. Its age has not been determined, and it will, perhaps, be found to be a tertiary or modern production.

SECTION IV.—THE NON-FOSSILIFEROUS STRATIFIED (OR METAMORPHIC) ROCKS.

1. Gneiss is the most abundant rock in this class, and is generally found reposing on granite. Its stratification is sometimes very distinct, but it is often so imperfect that it can scarcely be recognized. This is more frequently the case in the vicinity of granite on which it rests, and into which it insensibly passes. A large part of the material used for building purposes, under the name of granite, is obscurely marked gneiss. In all primary countries it is an abundant rock, occupying extensive districts, and sometimes forming mountain masses.

2. Mica slate lies next above gneiss, and is a very abundant rock. As it differs from gneiss only in the proportion of mica which it contains, and as the quantity of mica in it is very different in different places, it is often difficult to make the distinction between them. It also passes by insensible degrees into the argillaceous rocks. Many of the argillaceous rocks are found, upon close examination, to contain mica in minute scales in such abundance as to make it doubtful whether they ought not to be regarded as mica slates; that is, the metamorphic action by which argillaceous slate is converted into mica slate had proceeded so far, before it was arrested, that it becomes impossible to say whether the argillaceous or micaceous characters predominate.

3. Argillaceous slate.—The last rock of this series is a slaty rock, more or less highly argillaceous. It does not differ in lithological characters from the same rock in the higher strata. It is doubtful whether the roofing-slates should be considered as belonging to the metamorphic series or not. They have been subjected to a very high degree of metamorphic action, and yet strata intimately associated with them have, in occasional instances, contained fossils.

It is not easy to fix the exact upper limit of this series. The fossils are few, obscure, and seldom met with in the lowest fossiliferous series; and the transition is very gradual from the distinctly metamorphic to the fossiliferous rocks. This renders it impossible always to determine accurately the line of separation.

The gneiss, mica slate and argillaceous slate, have the order of superposition in which they are here named. They differ only in the amount of metamorphic action to which they have been subjected; and the gneiss which is most highly metamorphic has, by being the lowest, been most acted upon,—the mica slate less, and the argillaceous slate least. In a particular locality, however, the lowest rock which was subjected to these causes of change, instead of having been of such a character as to produce gneiss, may have been a limestone, and in that case the lowest metamorphic rock would be a saccharine marble. In another locality the lowest rock may have been a sandstone, which would be converted into quartz rock. Hence there may occur, in any part of the metamorphic series, crystalline limestone, quartz rock, hornblende slate, chlorite slate, and talcose slate; and any one of these rocks may be as abundant in any particular region, as gneiss, mica slate or argillaceous slate, is in another.

The metamorphic rocks occur in all countries where there has been any considerable amount of volcanic action, and their total amount is very great; but their stratification is so confused and contorted, their superposition so irregular, and denudations have been so extensive, that no estimate can be made of their thickness. They are, perhaps, equal to all the other stratified rocks.

SECTION V.—THE FOSSILIFEROUS ROCKS.

The strata of this system have a nearly vertical position, and consist principally of black, greenish and purple slates, of great thickness. Granular quartz rock, however, occurs in considerable quantity, and in this country two thick and important beds of limestone are found. These limestones are occasionally white and crystalline. Generally, however, as a mass, they are a dark, nearly black rock, with a network of lines of a lighter color. All the clouded marbles for architectural and ornamental purposes are from these beds, and our roofing and writing slates are all obtained from the argillaceous portion of this system.

The number of species of organic remains contained in this system is very small, and these, so far as discovered, belong to the annelida, with a few doubtful cases of mollusca. This system of rocks is found coming to the surface in a large part of New England, and the eastern part of New York, also in the western part of England and Wales.

Those geologists who deny the existence of this system consider these rocks as parts of the silurian system which have been most disturbed by subterranean forces, and most altered by proximity to igneous rocks. The annexed sketch (Fig. 7) will exhibit the relations here referred to. Certain portions of the silurian rocks are supposed to have been thrown into folds by the upheaval of the primary rocks. The plications nearest to the intrusive granite would be most altered. That part of the figure below the line a a represents the outcropping edges as they now appear, the upper portion of the folds having been removed by some abrading cause.

1. The Silurian System.—The following tabular arrangement exhibits the divisions of the system as recognized in England, in New York, in Pennsylvania and Virginia, and in Ohio.

This name, Silurian, was first used to designate the lowest well-characterized fossiliferous rocks in England. But it is now used to embrace the whole system as it occurs elsewhere. It is well exhibited in New York, both in consequence of its great development there, and because the whole system is only slightly acted upon by disturbing forces, so that the outcropping edge of each division extends over a large surface.

This system is of great thickness, amounting, in places where it is well developed, to twenty thousand feet.

The Champlain division commences with a quartzose sandstone, passing gradually into limestone, which is succeeded by a very thick argillaceous deposit, the Utica slate and Hudson River group. The Ontario division in the lower part is a mass of sandstone. Above this is the Clinton group, consisting of shales and sandstones. The most important part of this group, in an economical point of view, is a fossiliferous, argillaceous iron ore, coextensive with the group in this country, and is worked to supply a large number of furnaces. The last of the division is the Niagara group, which commences with a mass of shale, and becoming at length calcareous, it terminates in a firm compact limestone. This limestone has withstood the action of denuding causes better than the shales either above or below it. It therefore presents a bold escarpment at its outcrop, and occasions waterfalls wherever streams of water cross it. The falls of Niagara are formed by this rock. The Niagara limestone, in its extension westward, becomes the lead-bearing rock of Missouri, Iowa and Wisconsin. The Helderberg division is a succession of highly fossiliferous limestones, with the intervention of only occasional beds of grits and shales. One member of the series is the Onondaga Salt group. The water obtained from this group in New York annually furnishes immense quantities of salt. The Erie division consists of a thick mass of shales and sandstones.

The fossils of this system are very numerous, but consist mostly of the lower forms of animal life. Corals (Fig. 8. and 9) are abundant, and constitute in some places a large proportion of the limestones. The Crinoidea, or lily-shaped animals, consist of a jointed stem permanently attached, and bearing at the free extremity of the stem an expanded portion, which is the pelvis, or digestive cavity. The mouth is surrounded with a series of leaf-like tentacula, which serve the purpose of seizing and holding food. Fig. 10 represents the pelvis of one of the silurian fossils. The general character of the animal is better represented by Fig. 30. The most abundant fossils of this period are the lowest orders of bivalve mollusca (Fig. 11). The Cephalopoda are characterized by having the organs of locomotion attached to the head. The shell of several species is peculiar in being divided into distinct cells, or chambers (Fig. 12, b d), perforated by a tube (siphuncle a). These fossil shells are sometimes straight, as the Orthoceras (Fig. 13), or curved, as shown in the several forms of Fig. 14. The Trilobite was an articulated, crustaceous animal, having two lines along the back dividing it into three lobes, from which circumstance its name is derived. It is found in great numbers in the Silurian rocks (Fig. 15). In a few instances remains of fishes have been found, but they by no means characterize the system.

The fossils of this system are a few shells, a small number of vegetable species, and in particular localities the remains of fishes in great abundance. The system is characterized principally by fossils of this last kind. The fishes of this system have a cartilaginous skeleton, but are covered with plates of bone, which were faced externally with enamel. The jaws, which consisted of solid bone, were not covered with integument. The exterior bony covering seems to have been the true skeleton, as is, in part, the case with the tortoise. In some of the fishes of this period there is a wing-like expansion on each side of the neck, which has given them the name of Pterychthis (Fig. 17). In others, as the Cephalaspis, the plate of bone on the back is so large as to cover nearly the whole body, and make it resemble a trilobite (Fig. 18).

This system has an extensive geographical range. In England, it occupies a band of several miles in width, extending from the Welsh border northward through Scotland to the Orkney Islands. In this country, it forms the Catskill Mountains, in New York, and extends south and west so as to underlie the coal-fields of Pennsylvania and Virginia.

3. The Carboniferous System.—This system consists of three parts, distinguished by lithological and fossil characters.

The carboniferous limestone is a dark-colored, compact limestone, forming the base of the system, and reposing on the old red sandstone. Its thickness is from six hundred to one thousand feet, often with scarcely any intermixture of other rock; but it sometimes loses its character of a limestone, and becomes a sandstone, or conglomerate. It generally contains the ores of lead in considerable quantity, and from this circumstance has been called metalliferous limestone. In England it is the principal repository of these ores. In the Western States it is the upper portion of the lead-bearing strata.

Next above the limestone lies the sandstone, sometimes called millstone grit. It is generally drab-colored, but occasionally red. Its thickness is often equal to that of the limestone. Sometimes it is fine-grained and compact; but generally it is coarse-grained, and often passes into a conglomerate. It contains but few fossils, and those of vegetable origin.

The highest part of the system is the coal measures. They consist of beds of sandstone, limestone, shale, clay, ironstone and coal, occurring without much uniformity in their order of superposition. The coal measures have a thickness of about three thousand feet. The sandstones and limestones are not distinguishable from the sandstones and limestones in the lower part of the system. The ironstone either occurs in concretionary nodules, often formed around some organic nucleus, or it is an argillaceous ore, having a slaty structure. In either case, it consists of subordinate beds in the shale. The coal consists of several beds distributed through the measures. The beds vary in thickness from a few lines or inches to several feet. In a few cases beds have been found measuring fifty or sixty feet in thickness. The workable beds are ordinarily from three to six feet thick.

The carboniferous formation is very much disturbed by dikes, faults (Fig. 18; see also Fig. 50), and other dislocations. The amount of change of position in the strata, by faults, is very various; frequently but a few feet. In one case in England there is a fault of nearly a thousand feet. There is a case of dislocation in Belgium where the strata are bent into the form of the letter Z, so that a perpendicular shaft would cut through the same bed of coal several times.

The characters and order of superposition which have now been given may be regarded as the general type of the carboniferous formation. There are, however, several important modifications. 1. Beds of coal sometimes alternate with beds of millstone grit. Thus, in Scotland and in the north of England, this intermediate member of the system disappears, or, rather, is incorporated with the coal measures. The same is true, to considerable extent, in this country. 2. Sometimes the carboniferous limestone also disappears as a distinct member of the system, partly by becoming arenaceous, and partly by the intercalation of beds of coal. In this last case, the whole formation from the old to the new red sandstone becomes a series of coal measures. In this country the carboniferous limestone is found very generally to underlie the coal strata. 3. The fractures and faults, which were formerly supposed to be characteristic of the coal formation, are seldom found in the great coal-fields of this country, except in those of the anthracite coal of Pennsylvania; and even there they are much less common than in the coal-fields of Europe.

There are three principal varieties of coal, distinguished by the different proportions of bitumen which they contain. The common bituminous coal kindles readily, emits much smoke, and throws out so much liquid bitumen that the whole soon cakes into a solid mass. It contains about forty per cent, of bitumen. The second kind, or cannel coal, contains twenty per cent., and inflames easily, but does not agglutinate. The stone-coal, or anthracite, contains scarcely any bitumen, ignites with difficulty, emits but little smoke, and produces a very intense heat. The bituminous varieties are always found in the least disturbed portions of the coal districts; and the anthracite is found in the more broken and convulsed portions, where we may suppose that the subterranean heat has been sufficient to drive off the volatile bituminous part, and reduce it to the anthracite form. Hence the eastern Pennsylvania coal-fields, which lie near the principal axes of elevation of the Appalachian Mountains, furnish only anthracite; while the same coal-seams, in their extension to the western part of the state, are bituminous.

Where coal is quarried in large quantity, a shaft is sunk through the overlying strata to the coal-beds, and the coal is raised to the surface by steam power. After the coal has been quarried to some distance from the shaft, pillars of unquarried coal are left to support the overlying strata. Fatal accidents have sometimes occurred by the giving away of these supports. Over a large part of the coal-fields of the United States it has not yet become necessary to sink shafts. The quarrying is commenced at the outcrop of the coal-bed; and, till the cover becomes of considerable thickness, it has been found economical to “strip” off the overlying rock, rather than to work a subterranean gallery.

Brine-springs are often found in the coal measures of sufficient strength to be used in the manufacture of salt. This is now done to considerable extent in Ohio. In the valley of the Kenhawa river, Kentucky, the rocks of which belong to the carboniferous system, the brine is nearly saturated with salt; and in some of the borings they have even discovered beds of rock-salt of great thickness and purity.

There is no other part of the geological series so obviously connected with national prosperity as the coal formation. While a country is new, the forests furnish an abundant supply of fuel; but in the course of a few years these are consumed. This country will soon be principally dependent upon its coal-mines for fuel, even for domestic purposes; and, in carrying on the great branches of national industry, such as the smelting and working of iron, and in the formation of steam for the purposes of manufacture and transportation, we are already mainly dependent upon mineral coal. A nation which does not possess an abundant supply of this mineral, or which does not use it, cannot long maintain a high degree of national prosperity.

In these inexhaustible masses of coal, accumulated ages before the existence of the human race, is a most obvious prospective arrangement for securing our happiness and improvement. And this arrangement embraces not only the accumulation of a combustible material in such abundance, but also its juxtaposition with an equally inexhaustible accumulation of iron ore, and the limestone which is necessary as a flux in the reduction of the ore. So bulky and heavy materials as coal and iron ore could neither of them have been transported to any considerable distance for the manufacture of iron; and without the manufacture of iron on a large scale, the present operations in manufactures and transportation could never have been entered upon. A large proportion of the iron furnaces in this country, and nearly all of them in Great Britain, employ mineral coal for fuel, and obtain their ore from the beds contained in the coal measures.

The fossils of the coal measures are almost entirely of vegetable origin, and are very abundant. They are seldom found in the coal-beds, but in the strata of shale immediately above or below the solid coal.

The Stigmaria (Fig. 19) is found most abundantly, and in a large proportion of cases to the exclusion of every other form, in the lower shales. It consisted of a large dome-shaped mass, often three or four feet in diameter, with trailing branches, or roots, spreading off horizontally to a distance of twenty feet. In a few instances tree ferns have been found, petrified in a horizontal position, and being apparently a mere continuation of the stigmaria. Hence the stigmaria has been supposed to be the base of the tall tree ferns, the leaves of which so abound in the upper shales. If this is not the case, there are no forms of the existing flora of the earth analogous to the stigmaria. It is always found in connection with the coal-beds of the carboniferous formation, and never with the coal-beds which sometimes occur in the later formations.

The tree ferns (Fig. 20) attained a height of fifty or sixty feet, and a diameter of four feet. They have received the name of Sigillaria in consequence of the seal-like impressions (Fig. 21) with which the surface is covered, and which are the scars left where the fronds have fallen off. These fronds (fern leaves) are the most abundant fossil of the series. They are distinguished by some peculiarity in form, as the Sphenopteris (wedge-shaped fern leaf), Pachypteris (thick fern leaf), &c. (Fig. 22. and 23.)

There was another kind of Sigillaria (Fig. 24), in which the surface was fluted, and the markings are superficial, and occur on the ridges. It reached as great a size as the tree ferns, but to what general class of plants it belonged is still doubtful.

The Lepidodendron (scale-covered tree) (Fig. 25) is the fossil which most nearly resembled in general appearance our present forest trees. Specimens are found four feet in diameter and seventy feet in height. In botanical characters it resembled, in some respects, the trailing club-mosses, while in others it was very similar to the Norfolk Island pine.

The Calamite (Fig. 26) was a plant resembling, in its jointed and striated surface, the equisetum (rush), but was sometimes twelve inches in diameter.

The carboniferous formation exists more or less abundantly in all the great divisions of the earth. It occurs in nearly all of the countries of Europe. The largest deposits known are, however, in the United States; especially in the States of Pennsylvania and Virginia, and in Ohio.

4. The New Red Sandstone.—The lower division of this formation, called the Permian system, consists of a thick mass of sandstones, generally of a red color, with occasional alternations of argillaceous rock, succeeded by a series of magnesian limestones. The upper division, or Triassic system, is composed of a red conglomerate, a limestone which has received the name of Muschelkalk (shelly limestone), and a series of variegated marls and sandstones.

The ores of copper are found, to considerable extent, in this formation. The rich copper mines of Germany are in the magnesian limestone, or, as it is there called, Zechstein (minestone). The Lake Superior copper mines occur in a red sandstone formation, which will probably be found to belong to this system.

The salt-beds, salt springs, and beds of gypsum, are so, generally found in this rock in England, that it has been called by the English geologists the “saliferous system.” It is, however, found that in other countries these minerals occur in equal abundance in formations of an earlier and later date.