When the inclined position is produced by an uplift of the strata, along a given line, so that they dip in opposite directions, this line is called an anticlinal axis, as at Fig. 54. If, however, the strata are fractured along this line, as at b, the fracture becomes a valley of elevation.
If depression take place along a given line, as at c, the strata will dip towards this line, and it will be a synclinal axis. The depression will be a valley of subsidence. A synclinal axis would also be produced by an elevation of the strata, as at d and e, on each side of it, and the valley thus produced is one of elevation.
When successive sets of strata, as f and d, Fig. 53, are not parallel, they are said to be unconformable.
6. When the strata are subjected to displacement, they do not always take a merely inclined position, but are often contorted (Fig. 55), or folded together (Fig. 56). These folded axes frequently succeed each other for many miles. (See Figs. 7 and 82.) In the case represented by Fig. 56, if the highest portion has been removed, so that the line a b represents the actual surface, we shall have apparently a succession of deposits, of which those at b would be the newest, and the oldest would be found at a, when in fact the strata at the extremities are parts of the same layer.
It is probable that disturbances like those now mentioned have been taking place continually, in different places, from the earliest times. There have been no periods of universal disturbance, and none of universal repose. On the contrary, the periods of disturbance in one part of the world have been periods of repose in another. For example, the coal measures of Europe were much broken and disturbed before the deposition of the new red sandstone, and the close of the coal period was at one time supposed to have been a period of general convulsion. It is now ascertained that the principal coal-fields in this country were not much disturbed at that period, and have not been since.
SECTION III.—CHANGES OF ELEVATION AND SUBSIDENCE.
The continents, if we except the more rugged and broken portions, rise from the sea with an almost imperceptible ascent; and even the mountains have a much gentler slope than we are apt to suppose, so that a section of the earth parallel to the equator would be almost a perfect circle. The slope of a mountain, from its base to its highest point, rarely forms with the horizon an angle of as much as twelve degrees. In the following figure (57), A represents the peak of Chimborazo, B of Teneriffe, C of Ætna, and D of Mount Loa, the principal volcano of the Sandwich Islands. The highest mountains would be represented on a twelve-inch globe by an altitude of less than the one-hundredth of an inch above the level of the sea. But the rising and sinking of these masses, though so small compared with the dimensions of the earth, are yet geological changes on the largest scale.
1. The Elevation of Mountains.—Mountains have formerly been covered with the waters of the ocean. This is evident, in the case of some mountains, from the existence of stratified rocks reaching to the summits. The stratification could have been produced only by deposition from water. It is, moreover, evident from the existence of marine fossils, distributed through these strata, so abundantly, that they cannot be accounted for on any other hypothesis than that the animals lived and died where the remains of them are now found. These strata must therefore have formed the bed of the sea while the fossils were accumulating.
There is no direct evidence that the granitic mountain peaks were ever submerged. But there is reason for believing that the sedimentary strata which now occupy the lower slopes were, at the time of their deposition, continuous,—the igneous rock having subsequently broken through them,—so that the waters of the ocean once rested on the whole area which the mountain now occupies!
If the ocean could ever have been above its present level sufficiently to have covered all the sedimentary rocks, we might assume that the height of mountains has not been changed. But the level of the ocean cannot be subject to much variation. The total amount of water on the globe is always the same. If the continents and mountains were all submerged at once, and the waters were expanded by the highest temperature consistent with the liquid form, there would not be a change of level of more than two hundred and fifty feet. We may assume, then, that the ocean level has always been essentially the same that it now is. We must therefore conclude that the sedimentary rocks, and the mountains of which they form a part, have been elevated to their present position from the bed of the sea.
Different mountain ranges have been elevated at different periods. The silurian and carboniferous formations were deposited before the Alleghany Mountains, which they contributed to form, were elevated; while the new red sandstone and the cretaceous and tertiary formations were deposited subsequently to the upheaval. They are accordingly found at the base of the range, nearly horizontal, and have risen above the level of the ocean only as the continent generally has risen. The Pyrenees were elevated after the deposition of the cretaceous rocks, and have carried them up so that they appear at a high angle, while the tertiary rocks at the base are horizontal, as in the United States. The Andes have carried up the tertiary rocks with them, and their elevation must therefore belong to a recent period. It appears that they are even yet rising.
It has recently been shown that the Alps have been subjected to upheaval at several distinct periods. At the close of the silurian period they formed a cluster of islands. At the commencement of the tertiary period they became a mountain range, and at the close of that period they were thrown up some two thousand feet higher, to their present position. Nearly the same things will probably be found true of other mountain ranges, when their structure has been minutely studied.
The elevation of contiguous parallel ridges will necessarily leave intervening valleys of elevation. As mountain ranges generally consist of several such ridges, valleys of this description are numerous, and they are often of great extent.
It is obvious that there are mountains in the sea of as great height above the lowest valleys as the mountains of continents are above the level of the sea. If a new continent should hereafter be formed by the elevation of a large area of the bed of the sea, the existing mountains, now appearing in the form of islands, would partake of the general movement, and the new continent would have the same general diversities of surface as existing continents. The mountains would have existed long before the continent. It is therefore to be supposed that the mountains of the present continents were elevated before the continents, and that they stood for long periods as islands, exposed to the action of waves, tides, and marine currents.
2. The Elevation of Continents.—Continents have been elevated by so slow a movement that it has not generally been perceived, even when they have been peopled by nations advanced in civilization. And yet satisfactory evidence is always left of former sea-levels.
Almost every seaboard furnishes examples of beaches, evidently once washed by the sea, but now elevated more or less above high water.
At Lubec, near the northern extremity of the coast of Maine, barnacles[A] are found attached to the rocks eighteen feet above high water. The pilots at that place, and for a hundred miles north and south of it, speak of the ship-channels as diminishing in depth, though it is certain that they are not filling up. Such facts are to be explained only by supposing that the coast is rising.
[A] The barnacle is a marine animal, permanently fixed to the rocks, and live but a short time without being surrounded by sea-water.
Lakes are numerous throughout the northern portions of North America, which are receiving annually large quantities of sediment, and must ultimately become alluvial plains. Those of moderate depth, as Lake Erie, cannot require periods very protracted to fill them. Their continuance in such abundance indicates that the elevation of the continent to its present height is comparatively recent. This conclusion is confirmed by evidence of another kind. Throughout this region of lakes, beds of clay containing the remains of existing species of marine animals, are found at all elevations from the sea-coast, to the height of about four hundred feet, but not higher. These clay beds are very recent, and were deposited when the surface was four hundred or five hundred feet lower than it now is; and this amount of elevation has left the existing lakes scattered over the surface.[A]
[A] “It is remarkable that on the shores of the great lakes there are certain plants the proper station of which is the immediate neighborhood of the ocean, as if they had constituted part of the early flora of those regions when the lakes were filled with salt water, and have survived the change that has taken place in the physical conditions of their soil.”—Torrey’s Flora of the State of New York.
The following (Fig. 58) exhibits Europe as it was during the Silurian epoch, and Fig. 59 as it was at the commencement of the tertiary epoch. The land, as it then existed, is represented by the white surface, the present waters by the dark shading, and the land which has been reclaimed from the ocean by elevation since those periods by the lighter shading.
The whole southern part of South America, embracing an area equal to that of Europe, has been elevated within a very recent period; and some parts of it, if not all of it, are still rising. The shells found on the plains from Brazil to Terra del Fuego, and on the Pacific coast, at a height of from one hundred to thirteen hundred feet, are identical with those now inhabiting the adjacent seas. And “besides the organic remains, there are, in very many parts, marks of erosion, caves, ancient beaches, sand-dunes, and successive terraces of gravel,” all which must have resulted from the action of the waves at a period not remote. At Lima, articles of human skill peculiar to the original inhabitants of Peru were found imbedded in a mass of sea-shells eighty-three feet above the present sea level. The elevation on the Pacific coast has been in part by sudden uplifts of a few feet at a time; but it is found, from time to time, that there has been a change of level, amounting to a foot or more in a year, when there have been none of these sudden movements.
A considerable portion of Europe, reaching from North Cape in Norway to near the southern part of Sweden, more than a thousand miles, and from the Atlantic to St. Petersburg, more than six hundred miles, has been rising at the rate of about three feet in a century, for at least two centuries, and probably much longer. This change is proved by the occurrence, at considerable elevations above the sea, of shells now found in the Baltic; by rocks once sunken, now raised above the surface of the sea, and by ancient seaports having become inland towns. To determine the truth by actual measurement, the Royal Academy of Stockholm, about thirty-five years since, caused marks to be cut in the rocks along the coast, to indicate the ordinary level of the water. This is easily ascertained, as the Baltic is nearly a tideless sea. The present level of the sea, compared with that indicated by the marks before mentioned, leaves no doubt that the country is rising.
3. The Subsidence of Land.—Elevations can be shown to have taken place by fossils, and by other evidences of former sea levels which are left on the surface; but depressions leave but few indications of change of level. It is yet doubtful whether the depression is equal to the elevation; that is, whether the amount of land remains nearly constant, or whether there has been an augmentation of the dry land within the tertiary and recent periods. We are certain that the augmentation, if any, has not been equal to the elevation, for subsidences to a great amount are known to have taken place.
There are occasional instances of submerged forests seen at low tide, at some distance from the shore. There are several near the coast of England and Scotland, and near the coast of Massachusetts. They are but a few feet below low water, and do not indicate a subsidence of more than about twenty feet.
Numerous instances are on record of the sinking down of wharfs and buildings near the sea during earthquakes. Almost every violent earthquake is accompanied by a change of level. The changes of this kind which have been noticed are in seaport towns, because greater facilities are there afforded for detecting them, and because loss of property awakens attention to them; but there is every reason to suppose that these changes of level extend to great distances both into the country and into the sea.
An immense area in the Indian and Pacific Oceans, probably ten millions of square miles, is undergoing change of level. The lines A B and D G (Fig. 60) represent nearly the axes of depression; while an intermediate and two exterior parallel lines would represent axes of elevation. The evidence of these changes is found principally in the peculiarities of the wall of coral rock encircling the islands.
The following figures represent, in sections, modifications of form of the same island. The coral wall built up around the island by the polyps, from the depth of fifty, or at most of a hundred feet, is shown at c c (Fig. 61). If the island is elevated, this wall becomes a fringing reef (Fig. 62), b′ becoming the level of the sea, and the animal begins a new wall at the same depth as before. But if the island is gradually sinking, the wall is kept built up to the surface, and becomes a barrier reef (Fig. 63). A channel is thus left between the island and the reef, which, though gradually filling up with broken coral or other sediment, is generally deep enough for a ship-channel. If the island continue to subside till it disappears, and the coral wall is still kept at the surface, it then becomes an atoll, a circular coral island (Fig. 64), often of many leagues in diameter, beaten by the surf on the outer edge, but enclosing a quiet lake, which communicates only by occasional channels with the ocean.
The islands contiguous to the lines A B and C D (Fig. 60) are uniformly atolls, or are surrounded by barrier reefs, and are therefore subsiding; while the islands at a distance from these lines are surrounded by fringing reefs, which indicate that they are rising.
A well-authenticated instance of gradual subsidence is that of Greenland. The entire western coast, from its southern extremity to Disco Island, a distance of six hundred miles, has for the last two centuries been slowly subsiding. The dwelling-houses and places of worship built by the early European settlers are now in part or entirely submerged. The natives are said to be aware of the subsidence, and never build their huts near the sea.
4. We have thus seen that both elevation and depression may take place. There is reason to believe that these changes of level have, in some cases, been several times repeated. In one of the eastern ranges of the Andes, opposite to Chili, there is a mass of marine strata of five thousand feet in thickness. About the middle of the series there occurs a silicified forest. In one place a clump of coniferous trees was found of more than fifty in number, and a foot or more in diameter. The base of the strata must have been twenty-five hundred feet below the surface of the sea, in order to admit of the deposition of the first half of it. It was then elevated, so that a forest grew upon its surface. It was then depressed at least twenty-five hundred feet, more, to admit of the deposition of the subsequent strata, and the whole is now uplifted to form a mountain range of eight thousand feet in height.
The temple of Jupiter Serapis, near Naples, in Italy, was built near the sea, about eighteen hundred years ago. It was gradually submerged, and finally lost by the deposition of sediment nearly to the top of the columns. It was afterwards elevated, so as to be entirely above the level of the sea. The remains of the temple (Fig. 65) were afterwards discovered by the columns projecting a little above the ground. The sediment was removed to the depth of forty-six feet, when the workmen came to the base of the columns, and to a pavement seventy feet in diameter. In 1807 an artist was employed to take drawings of the ruin. The pavement was then above the level of the sea. Sixteen years afterwards the same artist found the pavement covered with water, and the depth has continued to increase since that time. It is considered that for the last forty years the depression has been three-fourths of an inch a year.
Instances enough have now been given to show how extensively the system admits of change. They are sufficient to justify us in searching for indications of great revolutions in past times, even where no such indications have as yet been discovered. They will serve as a key to many otherwise inexplicable phenomena, In order to the interpretation of such phenomena readily, we must cease to look upon these as exceptional cases, and regard them not only as facts, but as facts of frequent occurrence.
From the examples which have now been given, as well as from speculations upon the cause of these changes, it seems highly probable that all the surface of the solid portion of the earth, whether land or the bed of the sea, is undergoing changes of level. It may be so gradual that in the life of an individual it would be imperceptible, even where the best means of detecting it exist. These means are generally the works of man, and they are themselves so liable to change, that it would be scarcely possible to detect variations of level, which amount to but a few inches in a century.
If we admit that the relations of land and water have always been variable, it is impossible to arrive at any certain conclusion as to the amount, position or form, of the dry land at any former period. We may determine, with some degree of certainty, what portions of the present continents were submerged at particular epochs. Thus, we may infer that most of this country was submerged during the silurian period, from the great extent of the Silurian rocks; and, from the limited extent of the chalk formation in this country, we know that during the cretaceous period most of the continent was above the surface of the sea. But we have absolutely no data for determining what portions of the bed of the sea were at any time dry land.
It is supposable that the land has been principally confined to the equatorial regions at one period, and to the polar at another. At still a different period the land may have existed as islands scattered through a general ocean. These relations may, therefore, be assumed to have existed, if there are geological phenomena which best accord with such relations.
SECTION IV.—CHANGES ON THE SURFACE OF THE EARTH.
1. The principal changes of this class consist in the wearing down and removing immense quantities of the surface rock. The form in which the igneous rocks, of which the entire crust of the earth was originally composed, now appear, furnishes no assistance in judging of the amount of denudation which they have suffered. We can judge only from the amount of rock for which they have furnished the materials, and these are the whole sedimentary series which exist both as dry land and as the bed of the sea.
2. The sedimentary rocks have also been subject to great denudation; and we often have, in what is left, some indications of how much has been removed. One of these indications consists in the now level surface of those portions of country in which large faults exist. By the excavations for coal, in England, faults have been discovered of five or six hundred feet. At the time that they were formed, the surface must have presented precipitous escarpments (as represented by the dotted lines in Fig. 50) of a height equal to the dislocation; but the whole is now reduced to a general level (z z), denuding causes having removed the elevated portions.
The extent of valleys will often give some idea of the amount of denudation to which a region has been subjected. In the north-west of Scotland there is a succession of hills of about three thousand feet in elevation, consisting, for the upper two thousand feet, of horizontal strata of old red sandstone. (Fig. 66. We cannot conceive that these mountain masses were deposited in their present isolated form. The whole intervening spaces must have been filled with strata continuous with those by which the elevations are formed.[A]
[A] “I entertain little doubt that when this loftier portion of Scotland, including the entire Highlands, first presented its broad back over the waves, the upper surface consisted exclusively, from one extremity to the other, of a continuous tract of old red sandstone; though, ere the land finally emerged, the ocean currents of ages had swept it away, all except in the lower and last raised borders, and in detached localities where it still remains, as in the pyramidal hills of Western Rosshire, to show the amazing depth to which it had once overlaid the inferior rocks.”—Miller, Old Red Sandstone, p. 22.
A somewhat similar instance occurs in the Connecticut river sandstone, in the central part of Massachusetts. The following figure (Fig. 67) represents two mountains of 1 the sandstone, between which the Connecticut river flows. The dotted lines indicate a depth of one thousand feet of the rock which has been swept away. It is also thought that a bed of equal depth has been removed from this section southward, through the State of Connecticut, to the sea-coast.
3. Valleys, and even many of the larger valleys, are produced by the wearing down of the surface. The lower portion of the Connecticut valley is one of denudation, though in its upper part it is a valley of elevation, resulting from the upheaval of the Green and White Mountains. The water-courses from the mountains are transverse to the direction of the ranges, and generally consist of valleys of denudation. These valleys were no doubt originally fractures, produced while the mountains were rising. The fractures have been subsequently widened by denudation into valleys.
4. The rocky surface, beyond the fortieth parallels of latitude, and in the vicinity of glacier-producing mountains, is generally covered with grooves and striæ (Fig. 68), varying from several inches in depth to the finest perceptible lines. Rocks that are of a soft consistence, or which have been long exposed to atmospheric agents, seldom exhibit these marks, though there are probably few places, outside of the parallels before mentioned, where the rocky surface, if it has been protected from atmospheric decay, does not contain such grooving.
5. Another change at the surface consists in the formation of a soil; that is, of a superficial layer, of no great thickness, of earthy matter, a large proportion of which is always in a minutely divided state. In some instances it is common sediment, unsolidified; in others, it consists of the surface rock in a state of disintegration; but a large part of the soil within the region where the grooved surfaces are found consists of materials transported from a distance.
Soils are distinguished according to their predominant minerals, as siliceous, aluminous and calcareous. If siliceous matter is in excess, it will be a light, warm soil, and allow the water to pass through it too freely. If the clay predominates, the soil is cold, stiff, and too retentive of moisture. A proper admixture of these three ingredients constitutes the best soils. There are some other mineral ingredients essential to the productiveness of soils, but they are always in small proportion. In addition to the inorganic part which is common to the upper soil, and the subsoil, there is required, in order to render the upper layer productive, a large admixture of decaying animal and vegetable matter.
SECTION V.—CHANGES OF CLIMATE.
Our means of determining the climate of any former period consists in a comparison of the fossils of such period with the existing forms of life in warm and cold climates.
The earliest abundant vegetation consisted principally of ferns, rushes and mosses, and a larger growth was attained than is attained by any of the allied forms of the present time. We may infer that the circumstances under which these lower forms of vegetable life are now produced in the largest proportion, compared with other forms, and under which they grow to the largest size, are the circumstances approaching most nearly those under which the early vegetation was produced. These circumstances are found to be a position elevated but little above the level of the sea, a humid atmosphere, and the highest terrestrial temperature. Such facts favor the conclusion that during the coal period an ultra-tropical climate prevailed, and that the land existed in the form of low islands, thickly set in a general ocean.
The peculiar characters of some of the animal fossils, from the earliest fossiliferous to the tertiary series, indicate that a warmer climate prevailed during their formation than now exists. The remains of marine animals, such as the cephalopoda, are found in great numbers and in high latitudes, in a fossil state; but similar species, as the nautilus, now abound only between the tropics. The same is true of the crinoidea. Coralline limestone is also found in great abundance and in high northern latitudes; but the stone-producing coral now exists only in very warm seas. The remains of saurian reptiles are numerous in the oölite and Wealden; but all the larger recent species of the lizard tribe, such as the crocodile, are confined to the warmer regions of the earth.
A former warm climate in Siberia is indicated by the occurrence there of the remains of elephants. These animals were so abundant that their tusks are now collected as an article of commerce. The abundance and high state of preservation of these remains seem to preclude the explanation that they were conveyed there, from the present tropical regions, by any great geological convulsion. The species must therefore have inhabited the country, though the elephant is now found only between the tropics. The Siberian elephant was a different species from any now existing, and, unlike the recent species, had a covering of coarse hair. There is, however, no reason to conclude that it could endure a continued low temperature; and its sustenance would have been impossible, from the very stinted vegetation which that region now affords. We must therefore suppose that Siberia enjoyed, at the period when it supported these animals in such abundance, a tropical climate.
Most of the facts which go to prove a change of climate have been observed in the northern hemisphere; but the explorations in South America and New Holland furnish ground for believing that the geological phenomena of the two hemispheres are essentially alike, and that the indications of climate are the same for the same periods.
Such is, in general, the evidence in reference to climate; and it leads to the conclusion that a highly tropical climate prevailed in the temperate, and for some distance, at least, into the polar zones, in the early geological periods; while there is no reason for supposing that the tropical regions experienced a temperature too high for physical life to endure it. The climate of the earth was characterized then by a higher temperature than now, and by greater uniformity. This was the climate, with perhaps a gradual reduction of temperature, till the later portions of the tertiary period.
Before the close of the tertiary period, a change occurred, and probably a rapid one, to a more rigorous climate than now exists. The destruction of the elephant in Siberia was evidently sudden, and was followed by extreme cold; for the animals are in some cases entirely preserved in ice, and in so perfect a state that, when the ice which surrounds them becomes melted, the flesh is devoured by carnivorous animals. There are occasionally found, in the drift of the boulder period, shells similar to those of the Arctic regions, and in a condition to show that they have not been transported. The clay beds of the northern portion of the United States and of Canada were deposited during the last depression of that portion of the continent, and they contain the remains of marine animals identical in several instances with species now living, but confined to more northern regions. It must therefore be admitted that the interval between the middle tertiary and the modern era was one of great cold. It is generally referred to as the Glacial period.
Very considerable local changes of climate have also occurred within the historical period. Thus the mean temperature of the Alps has been so reduced that the ancient passes have in modern times become choked up with snow, and other passes have been sought,—a result, perhaps, of additional upheaval. It would seem that Siberia is now receiving a milder climate. The ice in which elephants have for centuries been imbedded has been slowly melting for at least thirty years.
SECTION VI.—ADVANTAGES RESULTING FROM GEOLOGICAL CHANGES.
1. The division of the general surface into land and water, as well as the diversified form of the land, the existence of mountains and low lands, and the consequent modifications of climate, the waterfalls, and the river-systems, constituting the drainage of continents, are all results of the process of upheaval.
2. A large part of the mineral substances employed for architectural and economical purposes are oceanic deposits, such as the marbles, slates, sandstones and mineral salt, and would have been inaccessible if they had not been elevated from the position in which they were formed. And the elevation of them above the bed of the sea would have exposed only the superficial layer, if they had not been either irregularly uplifted, as at e c (Fig. 69), or unequally worn down, as at b.
The granitic rocks, as they were formed below the aqueous rocks, must have remained unknown and useless, if they had not been brought to the surface, as at c, by the most convulsive efforts of nature of which we have any knowledge. Thus, natural mechanical forces have effected for man what the mechanical forces under his control would be entirely insufficient to accomplish.
3. It is by changes of this kind that we become acquainted with the geological structure of the crust of the earth. Mining operations have never extended to a greater depth than three thousand feet, while the inclined position of the strata exposes for examination, along their outcropping edges, e a c, the whole series, even to the primary rocks. The upheaval of the granitic rocks, and the removal by denudation of the overlying deposits, shows us the crystalline character which the earthy materials take, when subjected to pressure and cooled from fusion with extreme slowness. Thus we have, exposed to observation, the process of nature in the formation and modification of rocks for several miles in depth. Of the central portions, however, including by far the largest part of the mass of the earth, we have no knowledge whatever.
4. Springs, and the other means of obtaining water for domestic purposes, depend in part upon the inclined position of strata, and the broken and uneven condition of the surface, and in part upon the alternation of permeable and impermeable strata. If all the strata were porous, like the sandstones, the water which falls upon the surface would gradually settle through them to the level of the sea; or, if they were all impermeable, like the clays, the water would pass over the surface, and be collected in lakes or the ocean. As it is, the porous structure of the soil and of some rocks acts as a reservoir, from which the water is gradually discharged, and the intervention of impermeable strata prevents its taking a perpendicular direction downwards. Thus, if the stratum e b (Fig. 69) consists of porous rock, and the one below is impermeable, the water which is absorbed at e will appear at b as a spring. Or, if the line a d is a fracture, the water received at c may reappear as a spring at a. If the strata were perforated by boring at e till the porous stratum a is reached, the water will rise to the surface, constituting an Artesian well. An ordinary well consists of an excavation continued till a stratum is reached which is permanently saturated with water.
5. Most of the metallic ores which occur in the stratified rocks, with the exception of iron, are found in fractures or as dikes. Without these disturbances of the strata, the ores would have remained either sparingly diffused throughout the adjacent strata, or as a part of the melted mass at the volcanic centres. The ores and metals which are found in the primary rocks are accessible only by the bringing up of these rocks to the surface.
The fracturing, displacement, and elevation of the strata, attended, as is often the case, with the destruction of property and of the life both of man and the inferior animals, might, at first view, be thought an unnecessary, if not a wanton infringement upon arrangements already established. But the results which we have noticed, though by no means a full enumeration of the advantages resulting from geological changes, are sufficient to show that even the more violent disturbances to which the crust of the earth has been subjected constitute an important part of that series of adjustments which has rendered it a suitable abode for human beings. These changes are therefore neither useless nor accidental, but are essential parts of a wise and beneficent system.
CHAPTER IV.
OF THE CAUSES OF GEOLOGICAL PHENOMENA.
An exhibition of the composition and structure of the earth, together with an account, as far as there is reliable evidence, of the modifications which they have undergone, has been the object of the preceding chapters. They are mainly a collection and classification of observed facts. No reference has been made to causes or modes of operation, except in a few cases where it was necessary in order that a statement or description, might be intelligible.
If the facts have been given with sufficient clearness and detail to convey a correct general idea of the crust of the earth, we are prepared to inquire what are the agencies employed, and how they have operated in producing it. It is the province of the geologist to question every known power in nature, and to ascertain what geological effects each one is now producing; and, observing what effects are produced by given causes, he is to judge of the causes which have produced like effects in past geological periods.
Some of these causes are in their nature limited, and effects can be referred to them only within those limits. Thus, the congelation of water expands it by a certain proportion of its volume, and beyond that it can have no effect. But the expansive power of steam varies with the temperature; and hence the effects referred to it may be equally varied. Thus, we are not to expect exact uniformity of results in all past times, but the results will vary only as the circumstances vary upon which the operation of these causes depends.
Geological causes, in most instances, operate with extreme slowness; and therefore it will require a series of observations, continued for a long time, to ascertain what are the capabilities of these causes. But a single instance of their effects proves their capabilities thus far. Hence, one instance of the deposition of a stratum of salt in a salt lake; of the filling of a fracture with fluid lava; of a volcanic eruption, like that of Iceland in 1783; of the subsidence of a volcanic mountain, as that of Papandayang in Java; or of the rising of a large area of land, as in Sweden, as fully proves that natural causes exist capable of producing these effects, as if the effects were produced daily. As these effects increase in number, and careful observations are made and authentic accounts preserved, the means of correctly explaining geological phenomena will increase. The causes thus far known are Atmospheric Causes, Chemical Action, Organic Agency, and Aqueous, Aqueo-glacial and Igneous Action.
SECTION I.—ATMOSPHERIC CAUSES.
The oxygen of the atmosphere is capable of uniting with some of the constituents of rocks, by which their cohesion is weakened or destroyed. This is the cause of the rapid disintegration of some varieties of granite. The protoxide of iron which they contain is converted, by contact with the atmosphere, into the peroxide. Its volume is thus increased, and portions of the rock are separated from the mass. When granite or limestone contains sulphuret of iron, the oxygen of the atmosphere, in connection with moisture, combines with the sulphur, forming sulphuric acid, by which limestone and the felspar of granite are rapidly decomposed. Hence, a rock which contains an oxide or sulphuret of iron should not be used for architectural purposes.
Carbonic acid is another constituent of the atmosphere which operates as a decomposing agent. The water that falls from the atmosphere is charged with it, and thus becomes capable of dissolving calcareous rocks. Carbonic acid is thus indirectly the means of the rapid destruction of rocks of this class. It is also believed that carbonic acid enters into direct combination with some of the constituents of rocks, and particularly felspar; for it is found that in those countries where carbonic acid issues in great quantities from the earth, the rocks, especially those which contain felspar, disintegrate rapidly. Masses of many tons’ weight, which appear to be solid granite, after being broken are found to be in such a state of decay that fragments may be reduced to sand between the fingers.
The moisture of the atmosphere has some effect as a decomposing agent. Rocks which are exposed to frequent alternations of moisture and dryness soon crumble into fragments. Rain, falling upon the surface of rock, produces, mechanically, a destroying effect, which is not to be overlooked.
Variations of temperature, especially those alternations above and below the freezing point, have greater influence than any other cause in the destruction of rocks. When the water with which a rock is saturated congeals, the resulting expansion tends to enlarge the interstices, and thus to separate the particles of the rock. When the ice melts, the particles fail to resume the closeness of arrangement with which they were before packed. By frequent repetition of this action, the superficial portion loses its cohesion, and disintegrates. It is also found that in the region of perpetual snow the surface of the mountain masses is covered with rock in a disintegrated or fragmentary state, in greater abundance than below the snow line; but no explanation of this fact has yet been found.
In mountainous regions, electrical discharges and violent storms have some destroying effect. Winds have considerable power in changing the place of earthy matter in a disintegrated state. In deserts, the sands are carried in great quantities to great distances.
The causes now enumerated, when considered separately, and as acting for only limited periods of time, seem hardly worthy of notice; but when considered as operating conjointly, and for indefinite periods of time, they must have produced important changes on the surface of the earth.
From these causes, the surface and ornaments of castles and other ancient edifices, and of boulders, and all insulated rocks, are found to be decayed, and often to a considerable depth. It is from these causes that a soil is produced on every surface of rock which is not so exposed to the action of currents that the debris is removed as fast as it is formed. Hence it is, also, that a slope of detritus is formed at the base of every declivity, so that the ledge appears only at the highest points.
It is from a combination of these atmospheric causes that a large part of the sediment is furnished which brooks and rivers carry away. And when cohesion is not entirely overcome, it is so far weakened that other causes are much more effectual than they would otherwise be, in effecting the disintegration of rocks.
All those changes in which the action is molecular,—that is, between the molecules as such, and not between the masses,—including the effects of the imponderable substances, we regard as resulting from chemical agency.
Under the control of these molecular forces the crystalline rocks have taken their form; and if the crust of the earth could have remained in a fixed condition, in which these forces would have been in equilibrium, no further chemical action could have taken place. But, instead of being in a fixed condition, the present system is one of perpetual change. Various disturbances of this equilibrium of forces,—such, for instance, as the diurnal and annual changes of temperature at the surface, and the still greater secular changes of temperature at great depths,—will bring the chemical forces into operation. The mechanical disintegration of the crystalline rocks, and the deposition of them in strata independently of the chemical affinity of their particles, will give occasion for chemical changes,—that is, for a rearrangement of the particles in accordance with their affinities,—whenever any movement of the particles among themselves can take place. These movements take place, to a very great extent, under the influence of electrical currents, and of change of temperature, even while the masses retain their solid form.
Chemical affinity has exhibited itself on the largest scale in the formation of the various mineral species of which the crust of the earth is composed; but we may also refer to the same cause the formation of divisional planes in rocks, the concretionary arrangement, and mineral veins.
1. Divisional Planes.—It has before been stated, that the older rocks, in many cases, cleave freely in planes not parallel with the stratification. (See Fig. 48.) In some instances, in beds of lava, a similar cleavage exists, sufficiently perfect to allow of its use as a roofing material. In these cases, there must have been a rearrangement of the particles, so that their axes of greatest attraction would lie in parallel planes; the same arrangement which exists in mica and other crystalline substances, which have one and but one free cleavage.
A similar arrangement has sometimes taken place under such circumstances as to submit the process to more careful scrutiny. In the gold mines of Chili, the powder from which the gold has been washed is “thrown into a common heap. A great deal of chemical action then commences; salts of various kinds effloresce on the surface, and the mass becomes hard, and divides into fragments which possess an even and well-defined slaty structure.” When a portion of clay, worked into a paste with a very weak acid, is submitted to a weak voltaic action for several months, and then dried, it is found to have acquired a distinct though imperfect cleavage structure.
It appears, then, that both electrical currents and ordinary chemical action are capable of arranging the particles of an earthy mass into separable layers. We may then regard this change in the older rocks as an imperfect crystallization, and probably induced by electro-chemical agency.
It is also found that all rocks are divided into huge blocks by seams not parallel with the cleavage, and too regular to be considered as fractures. These seams bear an analogy to the secondary faces of crystals, which are never parallel to the cleavage.
2. Concretionary Formations.—There exist in many rocks concretions which differ from the mass of the rocks. In most of the tertiary clays there are small concretionary nodules, which contain more calcareous matter than the mass of clay around them. In the coal formation, the nodular iron ore consists of concretionary masses. In the chalk formation, nodules of flint abound, and generally in layers. In many of these cases, particularly in the clays and coal, the nodules have an organic nucleus, and, although concretionary, they retain the marks of stratification of the adjacent rocks. Hence they could not have been deposited in the form of nodules. There must therefore have been in the rock, though in the solid state, such motion among the molecules that particles of a particular mineral have separated from the mass and rearranged themselves in concretionary layers, yet so gradually as not to disturb the lines of original stratification.