The Elements of Geology; Adapted to the Use of Schools and Colleges Part 6

The lowest change of winch we can gain any information is the _formation of granite_. It will be shown hereafter that it has been in a melted state, and that it has taken its present form on cooling. But whether any considerable portions of the granitic ma.s.ses, or of the melted ma.s.ses now below the surface, have resulted from the fusion of stratified rocks, we have not the means of determining. It is, however, not improbable, that in the changes of level to which the crust of the earth has been subjected, the stratified rocks may have gone down so far as to become melted. At the same time, the melted rock which is thrown to the surface by volcanoes is subjected to the various destroying agencies by which it becomes sedimentary matter, to be deposited as mechanical strata. Thus, as the igneous rocks from below are brought up to furnish materials for mechanical strata, there must be an equal amount of depression of the mechanical strata towards the seats of igneous action. And if this change takes place more rapidly than the thickness of crust increases, then portions of the sedimentary rocks must be undergoing fusion.

Next above the granite an immense thickness of rock occurs, which exhibits, from its stratification and from the water-worn fragments which it contains, distinct evidence of its mechanical origin. And yet it is very different from the later mechanical formations. It is more highly crystalline; it has, to a great extent, a.s.sumed a cleavage distinct from the planes of stratification, and chemical affinity has been so far active as to produce new combinations, and give to them their peculiar crystalline form, as in the case of garnets, iron pyrites, &c. These strata also differ from those above them in containing no organic remains. It is not certain that organic life existed on the earth at the time when these rocks were deposited. Either it did not, or the evidence of it in the strata of that period has been obliterated. The changes have at least been sufficient to justify their being characterized as _metamorphic rocks_.

SECTION II.--CHANGES IN THE Ma.s.s OF THE STRATIFIED ROCKS.

1. The stratified rocks were deposited as mud or sand, and were at first in a yielding state. Most of these deposits have become _solidified rock_, such as limestone, clay slate and sandstone. The chalk of England is, however, but imperfectly consolidated, the great sandstone formation of New Holland is a friable ma.s.s easily disintegrated, and occasionally beds of clay in a plastic state are found as far down as the coal. Among the later rocks the solidification is less general, though there is some degree of hardening in all except the most superficial layers. The _fissile structure_ results from the solidification of the particles composing each layer separately.

2. Since the solidification of the strata, or perhaps in connection with it, there has been something of movement among the particles, resulting in mineral veins, conchoidal structure, &c. One of the most general changes of this kind is that by which a ma.s.s becomes separable into thin sheets, independent of the stratification, and not parallel with it.

This structure is represented by Fig. 48, in which the heavier lines are those of stratification, and the lighter of _cleavage_.

3. The strata have been everywhere more or less broken, and the _fractures_, nearly vertical, extend to groat depths. When a fracture reaches the surface, it often becomes a channel for water. It is thus widened by the erosion, the deepest parts become filled with debris, and it becomes a _gorge_, _ravine_ or _valley_.

[Ill.u.s.tration: Fig. 48.]

If the fracture does not come to the surface, it becomes a _cavern_. In limestone, caverns which are formed in this way are very frequent, and extend for many miles. There is generally a stream of water running through them, but not of sufficient volume to have produced the erosion which has been effected.

When the sides of the fracture are but little separated, some mineral often separates itself from the adjacent rock, and filling up the s.p.a.ce, reunites the broken parts. It is then called a _vein of segregation_ (Fig. 49, _a b_). But the fracture is more frequently filled with some volcanic rock injected from below. It is then a _dike_ (_c_ _d_), and may have a width of many rods, though it often diminishes in width till it is a mere thread. A dike of which the injected material is a metallic ore is a _mineral vein_.

[Ill.u.s.tration: Fig. 49.]

4. The uplifting force by which the fracture is produced has frequently raised the rock on one side higher than it has on the other. This is called a _fault_. (Fig. 50.) The unequal movements by which the fault is produced seem in some instances to have been repeated several times, and the grinding of the broken edges upon each other has polished and striated the sides of the fracture.

[Ill.u.s.tration: Fig. 50.]

[Ill.u.s.tration: Fig. 51.]

[Ill.u.s.tration: Fig. 52.]

5. Sedimentary rocks are often found with the planes of their strata more or less _inclined_. It is evident that they were not thus formed.

The depositions of sediment from water will always be horizontal, or, at most, only slightly inclined. But there is often evidence in the rock itself that its strata were once horizontal. It is frequently observed that vertical strata contain pebbles with their longer axes in the plane of the strata. (Fig. 51.) When these pebbles were deposited, the longer axes would take, on an obvious mechanical principle, a horizontal position. Their present vertical position must have resulted from a change in the position of the strata in which they are enclosed. The same thing is shown by the position of a petrified forest in the south of England, known as the Portland dirt-bed. Some parts of it are inclined at an angle of forty-five degrees. The position of the vegetable remains (Fig. 52) shows that when they were growing the surface was horizontal.

The line _b d_ (Fig. 53), on inclined strata which makes with the horizon the greatest angle, is called the _direction of the dip_. The angle thus formed (_a b d_) is the _angle of inclination_. When inclined strata come to the surface, the exposed edge, _b c_, is the _outcrop_, and the line of outcrop on a horizontal surface is called the _strike_ of the strata.

[Ill.u.s.tration: Fig. 53.]

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_.

[Ill.u.s.tration: Fig. 54.]

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_.

[Ill.u.s.tration: Fig. 55.]

[Ill.u.s.tration: Fig. 56.]

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 princ.i.p.al 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 aetna, and D of Mount Loa, the princ.i.p.al volcano of the Sandwich Islands. The highest mountains would be represented on a twelve-inch globe by an alt.i.tude of less than the one-hundredth of an inch above the level of the sea. But the rising and sinking of these ma.s.ses, 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 acc.u.mulating.

[Ill.u.s.tration: Fig. 57.]

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 a.s.sume 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 a.s.sume, 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 cl.u.s.ter 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[B] 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 s.h.i.+p-channels as diminis.h.i.+ng 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.

[B] 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 quant.i.ties 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.[C]

[C] "It is remarkable that on the sh.o.r.es of the great lakes there are certain plants the proper station of which is the immediate neighborhood of the ocean, as if they had const.i.tuted 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._

[Ill.u.s.tration: Fig. 58.]

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.

[Ill.u.s.tration: Fig. 59.]

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 sh.e.l.ls 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 ma.s.s of sea-sh.e.l.ls 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 sh.e.l.ls 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.