The Elements of Geology; Adapted to the Use of Schools and Colleges Part 8

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 ma.s.s 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 pa.s.s 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, const.i.tuting 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 ma.s.s 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 const.i.tute 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 cla.s.sification 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 circ.u.mstances 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 const.i.tuents 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 ma.s.s. 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 const.i.tuent 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 cla.s.s. It is also believed that carbonic acid enters into direct combination with some of the const.i.tuents of rocks, and particularly felspar; for it is found that in those countries where carbonic acid issues in great quant.i.ties from the earth, the rocks, especially those which contain felspar, disintegrate rapidly. Ma.s.ses 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 repet.i.tion 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 ma.s.ses 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 quant.i.ties 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.

SECTION II.--CHEMICAL ACTION.

All those changes in which the action is molecular,--that is, between the molecules as such, and not between the ma.s.ses,--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 ma.s.ses 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 circ.u.mstances 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 ma.s.s 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 ma.s.s 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 a.n.a.logy 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 ma.s.s of the rocks. In most of the tertiary clays there are small concretionary nodules, which contain more calcareous matter than the ma.s.s of clay around them. In the coal formation, the nodular iron ore consists of concretionary ma.s.ses. 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 ma.s.s and rearranged themselves in concretionary layers, yet so gradually as not to disturb the lines of original stratification.

[Ill.u.s.tration: Fig. 70.]

There are other instances, similar to the last in all respects, except that the segregated portion does not take the concretionary form. When gypsum is distributed in small proportion through a formation, there seems very little reason to doubt but that it is, by a molecular action, segregated from the strata in lenticular ma.s.ses, as at a (Fig. 70). Many of the limestone strata contain irregular aggregations of quartz. It is presumed that the siliceous and calcareous matter was deposited together as sediment, and that the aggregation has resulted from a movement among the particles similar to that by which the concretionary structure is produced.

The columnar structure of basalt seems to have resulted from a peculiar molecular action, at first resembling a concretionary arrangement, while the ma.s.s was cooling from a state of fusion. In experimenting to ascertain the cause of this structure, Mr. Watt fused in a furnace seven hundred pounds of basalt. When cooled, he found that "numerous spheroids had been formed, and that when two of them came in contact, they did not penetrate each other, but were mutually compressed and separated by a well-defined plane, invested with a rusty coating. When several met, they formed prisms." (Fig. 71.)

[Ill.u.s.tration: Fig. 71.]

3. _Mineral Veins._--The phenomena of veins are such that they cannot all be referred to the same cause. In some, the vein-stuff has been protruded as a dike, differing from ordinary dikes only in the accidental circ.u.mstance that it contains a metal or a metallic ore.

Mineral veins are not, however, generally filled by injection from below. It is found that those veins only are productive which have an east and west direction. But injected dikes run in all directions. The ore often varies in richness at different depths in the vein, or pa.s.ses into ore of some other metal. The ore also varies in kind and quality, according to the character of the rock through which the vein pa.s.ses.

These phenomena are best explained by supposing that the sediment of which the strata were formed contained the mineral substances of these veins in small proportion. After they were solidified, and fractures had been formed, the mineral substance was transferred by molecular action to the fissures, and deposited.

It was shown by the early experiments of Davy, that voltaic currents are capable of taking up mineral substances from their solutions, and removing them from one cup to another. It has been ascertained that in most mineral veins a proper apparatus will detect the existence of electric currents. It may be regarded as certain, that the unequal heating of different parts of the surface at the same time, by the sun, causes a vast current of feeble intensity to circulate around the earth once in twenty-four hours. The unequal distribution of heat below the surface may also produce currents subject to other laws. We should expect that these currents would take up the mineral substances diffused through rocks, and deposit them by themselves. It seems probable, therefore, that the molecular action, from which the segregation of metallic veins has resulted, was that of voltaic currents.

SECTION III.--ORGANIC CAUSES.

The effects of all organic causes in producing geological changes are inconsiderable, compared with those of inorganic causes. With the exception of the coral formation, the most important of these effects are those produced by human agency. We find examples of this agency in the distribution of animals and plants beyond the regions where they are indigenous; in the increased numbers of certain species, and in the diminution, if not extinction, of others; in the modifications of climate, dependent on the destruction of the forests and the cultivation of the soil; in controlling the course of rivers; in arresting by embankments the encroachments of the sea; in breaking up and changing the place of great quant.i.ties of rock by mining and engineering operations; and in the increased quant.i.ty of sediment furnished to streams by cultivating the surface, and thus preventing the protecting influence which the matted roots of trees and the smaller vegetables would otherwise have. Such effects, though attributable mainly to man, are produced in some degree by all other animals.

Besides these general effects, it is the existence of organic forms that has conferred on all the sedimentary rocks their fossiliferous character. _The records_ of the climate of each geological period, of the physical geography, of the vegetable productions, and of the animal forms by which the earth was peopled, consist in the remains of the living beings of these several periods, imbedded in the contemporaneous rock formations. But in the sediment deposited since the human era there must have been furnished both the remains of human beings and works of art, such as implements of labor and war, pottery, coins, fragments of s.h.i.+ps, &c.

Moreover, the _quant.i.ty of material_ which has been furnished by organic causes is by no means small. The coal-beds are the product of vegetable growth exclusively. We not unfrequently find strata of great extent consisting almost entirely of the sh.e.l.ls of molluscous animals, of the stems of encrinites, or of the s.h.i.+elds of microscopic animalcules.

But the most abundant rock which can be regarded as the product of animal organization is the _coral formation_. It consists of immense walls of coral limestone, separating either an atoll or the land of an island or continent from the open sea. The base of this wall has a width varying from a hundred feet to a mile or more, and the outer edge of it is at such a distance from the sh.o.r.e as to give a depth not much exceeding a hundred feet. Over this area of the bed of the sea, which forms the base of the wall, the coral polyp commenced its work.

Attaching itself in immense numbers over this area, it deposits calcareous matter from its under surface, and thus, by degrees, elevates itself towards the surface of the water, till it reaches a level a little above low-water mark. The height of the wall would not, with these conditions, exceed one hundred feet; but some hundreds of the islands surrounded by coral walls are gradually subsiding. The depositions of the polyps keep pace with the subsidence, so that this wall has reached an elevation from its base of a thousand feet, and in one instance of two thousand feet. (See Figs. 61, 62, 63, 64.)

Most of the islands of the torrid zone are thus surrounded with coral reefs, except a few where the cold polar currents reduce the temperature too low to admit of their growth. In one instance, along the north-east coast of New Holland, there is a coral reef, some twenty-five miles from the land, which has a continuous extension, excepting occasional inlets of no great depth, of a thousand miles. The reef along the island of New Caledonia is four hundred miles long. A large number of other reefs have a nearly equal extension. There is thus an area of several thousands of square miles covered to a great depth with this coralline limestone.

Some limestone formations of great extent among the older rocks were the work of similar animals. These lower forms of organization have, therefore, always been important geological agents, both in collecting the carbonate of lime from its solution in the waters of the ocean, and in depositing it as solid rock.

SECTION IV.--AQUEOUS CAUSES.