The Elements of Geology; Adapted to the Use of Schools and Colleges Part 7

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 sh.o.r.e. There are several near the coast of England and Scotland, and near the coast of Ma.s.sachusetts. 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.

[Ill.u.s.tration: Fig. 60.]

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 princ.i.p.ally 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 s.h.i.+p-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.

[Ill.u.s.tration: Fig. 61.]

[Ill.u.s.tration: Fig. 62.]

[Ill.u.s.tration: Fig. 63.]

[Ill.u.s.tration: Fig. 64.]

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 wors.h.i.+p 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 ma.s.s 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.

[Ill.u.s.tration: Fig. 65.]

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 princ.i.p.ally 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 a.s.sumed 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 princ.i.p.al changes of this cla.s.s consist in the wearing down and removing immense quant.i.ties 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 a.s.sistance 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 ma.s.ses were deposited in their present isolated form. The whole intervening s.p.a.ces must have been filled with strata continuous with those by which the elevations are formed.[D]

[D] "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 Ross.h.i.+re, to show the amazing depth to which it had once overlaid the inferior rocks."--_Miller, Old Red Sandstone_, _p. 22_.

[Ill.u.s.tration: Fig. 66.]

[Ill.u.s.tration: Fig. 67.]

A somewhat similar instance occurs in the Connecticut river sandstone, in the central part of Ma.s.sachusetts. 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.

[Ill.u.s.tration: Fig. 68.]

4. The rocky surface, beyond the fortieth parallels of lat.i.tude, and in the vicinity of glacier-producing mountains, is generally covered with _grooves_ and _striae_ (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 pa.s.s 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 const.i.tutes 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 princ.i.p.ally 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 circ.u.mstances 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 circ.u.mstances approaching most nearly those under which the early vegetation was produced. These circ.u.mstances 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 lat.i.tudes, 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 lat.i.tudes; but the stone-producing coral now exists only in very warm seas. The remains of saurian reptiles are numerous in the oolite 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 coa.r.s.e 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, sh.e.l.ls 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 pa.s.ses have in modern times become choked up with snow, and other pa.s.ses 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, const.i.tuting 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_.

[Ill.u.s.tration: Fig. 69.]

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.