The Elements of Geology; Adapted to the Use of Schools and Colleges Part 12

So far as observations have been made, the elevation of mountains seems not to be gradual, but spasmodic; and yet the elevating force probably acc.u.mulates constantly and uniformly. The repressing force consists of the weight of the strata above, which may be regarded as constant, and their strength, which is variable. When the elevating force becomes greater than both the repressing forces, the crust is fractured. The strength of the strata then becomes nothing, and the repressing force is the weight alone. The elastic ma.s.s below at once expands, and the requisite s.p.a.ce is furnished by the uplifting of the strata along the line of fracture. As the ridge of lava which fills this additional s.p.a.ce cools, it recloses, in part, the original fracture, and the repressing force again consists of the two elements,--weight and strength. There will therefore be no further elevation till the elevating force is again superior to these two forces. Thus the elevating force, though it may acc.u.mulate at a uniform rate, will manifest itself only at considerable intervals.

As the acc.u.mulation of lava along the line of fracture is the cause of the upheaval, every mountain must have a central granitic axis.

Sometimes this granitic ma.s.s is pushed up through the fissure, as in the case of Mont Blanc. At other times, the stratified rock, which formed the original surface, is carried up so as to form the surface rock nearly to the top. In either case, the strata are lifted along the line of fracture, and left in an _inclined position_. In this position the older rocks are always found, wherever there has been any considerable amount of igneous disturbance.

In some instances, the additional s.p.a.ce required by the expansion of the igneous ma.s.s below is furnished, not by the uplifting of the strata, but by their compression into folds between two lines of upheaval. The igneous rock is elevated but little above the stratified through which it had burst; but the stratified rocks have taken the undulatory form, and the widening of the igneous ma.s.s along the lines of fracture has compressed the undulations, until the planes of _the strata have become vertical_. Fig. 82 will give an idea of the successive changes by which the vertical position of the strata has been produced.

[Ill.u.s.tration: Fig. 82.]

The force by which mountains are elevated being the elasticity of the vapor diffused through the subjacent lava, it may happen, if the lava have a high degree of fluidity, that this vapor will collect in large ma.s.ses, and rise as far as the lava is in a fluid state. The irregular flow of lava from craters during an eruption is undoubtedly due to the rapid ascent of such steam bubbles through the lava. Such an acc.u.mulation of vapor under a mountain ma.s.s, if it cannot escape, would support it as long as the temperature remained unchanged. But, upon a reduction of temperature, the ma.s.s which had been upheaved by it would be unsupported, and liable at any time to sink. Instances of _subsidence_ on a comparatively small scale will admit of explanation in this way. Papandayang, one of the loftiest volcanic mountains of Java, sunk down four thousand feet in the year 1772. The area engulfed was sixteen miles long and six broad. The crater of Kilauea, in one of the Sandwich Islands, was evidently formed in this way. It is situated on the side of a mountain, and consists of a chasm eight miles in circ.u.mference and a thousand feet in depth. Liquid lava can always be seen boiling in the small craters at the bottom; and at times it rises so as to overflow them, and fill the chasm to within four hundred feet of the top, when lateral subterranean pa.s.sages are opened, by which it is discharged. The same explanation--a depression of the central portion--may be given of the formation of the large craters in the Canary and Grecian islands. It is also probable that Lake Avernus and others, in Italy, and some in Germany, have had a similar origin.

The subsidence of Papandayang is of importance as a historical fact; and it is not at all unreasonable to suppose that larger chasms of great depth were also sudden subsidences of a similar character. Lake Superior has a depth considerably greater than the elevation of its surface above the level of the sea. The bottom of the Dead Sea is two thousand six hundred feet below the surface of the Mediterranean. And at one place in the Atlantic Ocean a sounding was attempted with more than six miles of line, without reaching bottom. These sunken areas, however, though of great extent, occupy only an insignificant portion of the entire surface of the earth.

6. _The Elevation of Continents._--The causes of change of level which have been given will not explain those _slow vertical movements_ which are now taking place in Greenland and the north of Europe, or those by which the present continents have been elevated and the bed of the sea depressed. Any cause which will account for these movements must be one operating for long periods, under large areas, and with great uniformity.

The cause which fulfils all these conditions most satisfactorily is a _variation of temperature_ in the ma.s.s of rock underlying the portion of the surface whose level is changing. It has before been shown that the temperature increases as we descend below the surface; but there is also reason to suppose that it undergoes great variations. The volcanic grits interstratified with the silurian rocks of England show that at the silurian period volcanic fires were active below that portion of the surface. When the early fossiliferous rocks of this country were deposited, the Alleghany Mountains had not been elevated; but before the tertiary period they had taken nearly their present form. Some portion of the intermediate period was therefore one of volcanic upheaval. The trappean rocks are also evidence of intense volcanic action existing here. France, during the tertiary period, was a highly volcanic country; but all volcanic activity has now subsided. The Andes have been mostly elevated since the tertiary period, and are still rising. It is evident, then, that at different periods volcanic heat may vary from its highest to its least degree of activity, below any portion of the earth's surface.

This variation of temperature must be followed by variation of volume of the earth's crust; that is, it _must produce expansion or contraction_.

Experiments have been made, under the direction of the United States government, to determine the expansion of the several kinds of rock used in our public works. It was found that granite expands nearly one two hundred thousandth of its length for every degree of increased temperature, limestone somewhat more than that, and sandstone about twice as much. Taking the expansion of the granite as the basis of calculation, and supposing the crust for a hundred miles in thickness to be undergoing change of temperature, there would be a resulting difference of level exceeding two and a half feet for each degree of change in temperature, or more than two thousand five hundred feet for a change of one thousand degrees.

This calculation is made upon the supposition that the law of expansion is the same for all temperatures, and that no new conditions are introduced at high temperatures by the presence of aqueous particles. We know, however, that solids expand more rapidly at high temperatures than at low, and the elasticity of aqueous vapor at high temperatures must increase the rate of expansion of the rock through which it is diffused.

Although we are not able to introduce, numerically, the effect of these two circ.u.mstances, yet it is obvious that they must be considerable.

The mean elevation of land above the level of the sea is about nine hundred feet, the mountain ma.s.ses above that level not being included; and the estimated mean depth of the ocean, not including its chasms, does not exceed two thousand six hundred feet. The _total elevation of the continental ma.s.ses_, for which it is necessary to account, does not therefore exceed three thousand five hundred feet. This amount of vertical movement may evidently be produced by the expansion and contraction resulting from changes of temperature.

These changes of level must, however, be very gradual. Any diminution of temperature must result from the transfer of heat to the surface; and the conducting power of rocks is very imperfect. The lava in a crater is often so cooled on the surface that it can be walked on, while but a few feet below it is still liquid. Lava currents continue in gradual motion long after the surface is nearly cold. This was the case with one of the currents from aetna for more than nine months after its eruption, and with another for ten years. Humboldt visited Jorullo forty years after it was thrown up, when the lava around the mountain was still in a heated state, the temperature in the fissures being on the decrease from year to year; but twenty years after its ejection the heat was still sufficient to light a cigar at the depth of a few inches. If so long a period is insufficient to solidify a comparatively small quant.i.ty of melted rock when the circ.u.mstances for cooling are most favorable, we may well suppose that centuries would be required to abstract sufficient heat from the earth's crust to produce any material change in the areas of continents.

If this account of the elevation and subsidence of continents is correct, it would seem that they ought to be constantly undergoing change of level. And their _apparent stability_ may be regarded as an objection to it. If in any place there is absolutely no vertical movement, then those conditions must exist in which, for the time being, there is no change of temperature.

But it is doubtful whether there ever is absolute stability of any portion of the surface for long periods of time. Of the minor vertical movements of the interior of continents, there can, from the nature of the case, be no evidence whatever. Changes of level, where they are known to be taking place, are so slow, that they are hardly perceptible in the period of a human life. Such changes had been going on for centuries in Sweden before they were suspected. As accurate observations have increased in number, and historical records become available, it is becoming known that a very large amount of the seaboard is undergoing change of level. It becomes probable, then, that these extremely slow changes of level are constantly and everywhere taking place.

That portion of the crust of the earth const.i.tuting the present continents, being further removed from the centre, would part with its heat more rapidly, and receive heat from the central ma.s.s more slowly, than that portion which at present const.i.tutes the bed of the sea. The continents are therefore in a situation to undergo contraction and depression, and the bed of the sea is most favorably situated for rising. If the distribution of water through the ma.s.s has any influence in promoting its expansion, then the bed of the sea would receive this supply most abundantly, and the continents the least so. We see, then, in nature, those provisions for an alteration of level, which, from the character of the several rock formations, we know to have taken place.

When any portion of the earth's surface is covered with the sea, the conditions exist which will at length elevate it. When it becomes dry land, the conditions exist which will in time depress it below the level of the ocean. Hence, those impressions in regard to the land, as stable beyond the possibility of change, we ought to abandon; and those vertical movements, which, when proved, we are accustomed to regard as extraordinary, we shall, at length, consider as only particular instances of one of the most general laws of nature.

7. _Variations of Climate._--The only sources of heat by which climate can be affected are the sun and the heated interior of the earth.

If the former melted condition of the entire ma.s.s of the earth be a.s.sumed, the temperature of the surface must have been increased, by conduction of heat from within, for long periods after the superficial stratum had become solid. It is, however, susceptible of proof, that the present climates are not sensibly affected by interior heat, though at a little more than a mile below the surface the temperature is equal to that of boiling water. At any time, therefore, after the waters had become condensed, collected into oceans, and become sufficiently cool to support the animal life of which the remains are now found, it is not probable that the climate was, to any considerable extent, influenced by the heat conducted from the interior.

Still, there have been great changes of climate since those early organic forms existed; and, since we have no ground for supposing that the temperature of the sun's rays has suffered any reduction, we have to inquire whether the means of retaining the heat from the sun could at any time have been different. _The relative position of land and water_ depends, as we have seen, upon igneous causes, and has been very different at different times. We shall find that climate must have been greatly modified by these changes; for the land radiates and absorbs heat freely, and water possesses this power in a very low degree.

Let us suppose the zone comprised between the tropics to be occupied by land, and the portions without these limits to be covered with water.

Under these conditions, the land, having a nearly vertical sun the whole time, would acc.u.mulate heat to a degree scarcely compatible with the existence of animal life. This is sufficiently proved by the oppressive tropical climates of the present time, influenced as they are by polar lands and contiguous seas.

Under the same conditions, the sea would be heated by contact with the land, and the heat would be distributed by marine currents to the polar regions. But the water thus distributed would not part with its heat, because it has but little radiating power, and nowhere comes in contact with polar land. It follows, then, that both land and water would be subjected to a very high temperature.

But, if we suppose the land confined to the polar regions, and the sea to the equatorial, the opposite results would follow. The equatorial sea would absorb but a small proportion of the solar heat which would be thrown upon it. The land would receive the sun's rays too obliquely to receive much elevation of temperature, as the present polar climates show. Hence, the temperature of the earth would differ but little from that of the planetary s.p.a.ces, which is fifty-eight degrees below zero, a temperature too low to allow of any considerable development of organic life.

These are the conclusions to which we are led by considering the different powers of land and water to absorb and radiate heat, and we shall find that the existing climates are in accordance with these conclusions. America has a lower temperature than Europe in the same lat.i.tudes. It has also a smaller proportion of land in the equatorial regions, and a greater proportion in the north polar regions. The eastern continent is colder in Asia than in Europe in the same lat.i.tudes. It has also less equatorial and more polar land. The southern is colder than the northern hemisphere at equal distances from the equator. There is also less land near the equator on the south side, and probably as much land around the south as the north pole.

Hence, we see that there may have been such a relation of land and water as to account for all the variations of temperature which are known to have existed. We cannot say that such actually has been the case. We can tell, with some degree of accuracy, what portions of the present continents were land at the several geological periods; but three-fourths of the surface of the earth is covered with water, and of the condition of this portion during those periods we have no means even of conjecturing. We can only say, that, by the operation of known causes, the relative position of land and water may have been such as to produce the climates known to have existed at former periods of the history of the earth.

INDEX.

Page A.

Abundance of vegetable products of the coal period, 59 Acc.u.mulation of vegetable matter, 117 Actinolite, 15 Action of internal heat, 128 Action of waves, 107 in forming harbors, 108 Advantages of geological changes, 91 aetna, 26, 73 Agate, 14 Age of rocks, doubtful-- from change of lithological character, 61 from distance, 61 from disturbance, 61 Alternation of coa.r.s.e and fine material, 115 Aluminium, 12 Amethyst, 14 Amygdaloidal structure, 17 Ancient volcanic rocks, 29 Andes, granite veins in, 25 Angle of inclination, 71 Anoplotherium, 55 Anticlinal axis, 71 Aqueous causes, 103 Aqueo-glacial action, 120 Argillaceous schist, 20, 31 Arrangement of materials in the crust of the earth, 21 Artesian wells, 92 Asbestos, 15 Atmospheric causes, 95 Atolls, 81 Augite, 15 Auvergne, volcanic district of, 28

B.

Basalt, 18 Bed of the sea-- sunken areas in the, 142 why elevated, 145 Belemnites, 52 Breccia, 19 Brine springs-- in Silurian rocks, 35 in the carboniferous formation, 43 in the new red sandstone, 47

C.

Calamite, 47 Calcium, 13 Cambrian system, 32 Carbon, 11 Carbonate of lime, 15 Carbonate of magnesia, 19 Carbonic acid a cause of disintegration of rocks, 95 Carboniferous formation, 39 essential to national wealth, 43 extent of, 47 a prospective arrangement, 43 faults in, 41 not always disturbed by faults, 42 Carboniferous limestone, 39 sometimes becomes a coal-bearing rock, 42 fossils of the, 40 Carnelian, 14 Cause of internal heat, 128 Cause of stratification, 114 Caverns, 69 Cephalaspis, 39 Cephalopoda, 36 in oolite, 50 Chalcedony, 14 Chalk, 52 Changes of climate, 88 how produced, 146 Changes in the crust of the earth, 67 of temperature a disintegrating agent, 96 at the surface, 85 Chemical action, 97 in solids, 99 in crystallization, 97 Chlorine, 12 Chlorite, 15 Chlorite slate, 32 Cla.s.sification of rocks, 21 Clay, 19 Clay slate, 20 Cleavage structure, 68, 98 Coal, 16 varieties of, 42 mode of quarrying, 42 origin of, 116 conversion of vegetable matter into, 117 now forming, 118 Coal measures, 131 fossils of the, 44 Coal plants, tropical character of, 88 Coal and iron a.s.sociated, 43 Clouded marble, 33 Columnar structure, 18, 99 Compact limestone, 19 Concretionary formations, 99 Conglomerate, 19 of old red sandstone, 38 Connecticut valley-- one of denudation, 87 trap of, 30 Continents-- mean elevation of, 144 total elevation of, 144 elevated gradually, 144 why depressed, 145 Contorted strata, 72 Coral formation, 81, 102 extent of, 102 Coral reefs-- fringing, 81 barrier, 81 Coral rag, 49 Corals in silurian rocks, 35 Copper mines of Lake Superior, 47 Creation, a progressive work, 62 Cretaceous formation, 52

Cretaceous formation, fossils of, 52 geographical range of, 53 Crinoidea in Silurian rocks, 36 Crust of the earth, 16 expansion and contraction of, 143

D.

Delta deposits, 114 Denudation of igneous rocks, 85 Denudation of sedimentary rocks, 85 Denudation produced by earthquake waves, 136 Deposition of sediment, 113 Diluvium, 20 Dike, 69, 133 Divisional planes, 68, 98 Dolomite, 13, 19 Drift, 20 extent of, 53 connected with striated surface of the rocks, 54 connected with subsidence, 54, 126

E.

Earth in a state of change, 97 Earthquakes, 130 wave-like motion of, 136 Earthquake waves, rocks s.h.i.+vered by, 137 Effect of atmospheric agencies, 95 Electrical discharges, effect of, 96 Elementary substances, 11 Elevation and subsidence, 73 Elevation and subsidence several times repeated, 82 Elevation of mountains, 73 cause of, 139 spasmodic, 140 gradual, 75 Elevation of different mountains at different times, 75 Elevation of continents, 76 cause of, 142 Elevation of North America, 76 Elevation of the coast of Maine, 76 Elevation of Europe, 77 Elevation of South America, 78 Encrinites, 36, 50

F.

Fault, 41, 69 Felspar, 14 Filling up of lakes, 76 Fingal's cave, 30 Fissile structure, origin of, 68 Flint, 14 in chalk, 52, 99 Fluorine, 12 Folded axes, 61, 72 Formation of soils, 87 Fossils-- definition of, 57 how preserved, 57 mineralization of, 58 use of, 60 order in which animals appeared, shown by, 58 animal and vegetable, created together, 59 as a record of climate, 88, 101 Fossiliferous rocks, 32 cla.s.sification of, 32 Fractures, 42, 68, 130 opening downward, 133

G.

Garnet, 16 Geological causes, how far uniform, 94 Geological causes, slow operation of, 95 Geological investigations aided by displacement of strata, 92 Geological periods, prolonged, 63 shown by amount of strata, 63 shown by duration of species, 64 shown by amount of organic matter, 64 shown by microscopic acc.u.mulations, 65 Geology and Revelation, 65 Giant's Causeway, 30 Glacial period, 90 Glacial theory, 124 Glaciers-- how formed, 120 cause of motion, 121 when they decrease, 122 earthy matter on them, 122 lateral moraines, 123 surfaces grooved by, 123 terminal moraines, 123 Gneiss, 18, 30 Gorge, 69 Graham Island, 131 Granite, 16 varieties of, 16 thickness of, 23 structure of, 23 formation of, 67 igneous origin of, 138 Granite veins, 24 in granite, 24 Granite of different ages, 25 Granitic axes of mountains, 24, 140 Greensand, 19, 52 Greenstone, 18, 30 Grooved surfaces of rock, 54, 87, 126 Gypsum, 15 in new red sandstone, 47 beds, how produced, 99

H.

Hall, Sir James, experiments, 135, 139 Heterocercal tails of fishes, 48 h.o.m.ocercal tails, 48 Hornblende, 14 Hornblende slate, 20, 32 Hydrogen, 11 Hypersthene, 15 Hypersthene rock, 17, 25

I.