Part 5

The rocks here considered may be, and, as we have already seen, sometimes are, deposited in a purely chemical way, without the aid of life; and it is important to observe that in no case do the organisms make the silica and carbonate of calcium of their skeletons, but they simply appropriate and reduce to the solid state what exists ready made in solution in the sea-water. These minerals, and others, as we know, are produced by the decomposition of the rocks of the land, and are being constantly carried into the sea by rivers; and, if there were no animals in the sea, these processes would still go on until the sea-water became saturated with these substances, when their precipitation as limestone, etc., would necessarily follow. Hence it is clear that all the animals do is to effect the precipitation of certain minerals somewhat sooner than it would otherwise occur; so that from a geological standpoint the differences between chemical and organic deposition are not great.

This section of our subject may be summarized as follows: Animals and plants contribute to the formation of rocks in three distinct ways:—

1. During their growth they deoxidize carbon dioxide and water, and reduce to the solid state in their tissues carbon and the permanent gases oxygen, hydrogen, and nitrogen; and after death, through the accumulation of the half-decayed tissues in favorable localities,—marshes, etc.,—these elements are added to the solid crust of the earth in the form of coal and bitumen.

2. During the decomposition, i.e., oxidation, of the organic tissues, the iron existing everywhere in the soil is partially deoxidized, and, being thus rendered soluble, is removed by rain-water and concentrated in low places, forming beds of iron-ore.

3. Through the agency of marine organisms, certain mineral substances are being constantly removed from the sea-water and deposited upon the ocean floor, forming various calcareous and siliceous rocks.

I now bring our study of the aqueous or superficial agencies to a conclusion by noting once more that the great geological results accomplished by air, water, and organic matter or life are: (1) Erosion, or the wearing away of the surface of the land; and (2) Deposition, or the formation from the débris of the eroded land of two great classes of stratified rocks,—the mechanically formed or fragmental rocks, and the chemically and organically formed rocks.

II. IGNEOUS AGENCIES.

We pass next to a very brief consideration of operations that originate below the earth’s surface. The records of deep mines and artesian wells show that the temperature of the ground always increases downwards from the surface; and the much higher temperatures of hot springs and volcanoes show that the heat continues to increase to a great depth, and is not a merely superficial phenomenon. The observed rate of increase is not uniform, but it seldom varies far from the average, which is about 1° Fahr. per 53 feet of vertical descent, or, in round numbers, 100° per mile. This rate, if continued, would give a very high temperature at points only a few miles below the surface; and until within a few years the idea was generally accepted by geologists that the increase of temperature is sensibly uniform for an indefinite distance downward; that in the central regions of the earth the temperature is far higher than anything we can conceive, and that everywhere below a depth of 20 to 40 miles the temperature is above the fusing-point of all rocks; and hence that the earth is an incandescent liquid globe covered by a thin shell or crust of cold, solid rock.

Our limited space will not permit us to enter into a discussion of the condition of the earth’s interior, and I will merely point out in a few sentences the position occupied by geologists at the present time. The reasoning of Thompson has shown that the temperature cannot increase downward at a uniform rate, but at a constantly and rapidly diminishing rate; and that everywhere below a depth of 300 miles the temperature is probably sensibly the same, and nowhere, probably, above 8000° to 10,000° Fahr.

Unlike water, all rocks contract on solidifying and expand on melting, and consequently the high pressures to which they are subjected in the earth’s interior—10,000,000 to 20,000,000 pounds per square inch—must raise their fusing-points enormously, and the probabilities are that they are solid, in spite of the high temperature. But Thompson and Darwin have shown us farther that the phenomena of the oceanic tides could not be what they are known to be if the earth were any less rigid than a globe of solid steel; while Hopkins has proved that the astronomical phenomena of precession and nutation could not be what they are if the earth’s crust were less than 800 or 1000 miles thick. Putting these considerations together, geologists are almost universally agreed that, while the earth has an incandescent interior, it is still continuously solid from centre to circumference, with the exception of a thin plastic stratum at a depth not exceeding 40 or 50 miles, which forms the seat of volcanic action.

The earth is not only a very hot body, but it is rotating through almost absolutely cold space, and therefore must be a cooling body. But, except at the very beginning of the cooling, the loss of heat has gone on almost entirely from the interior; and since cooling means contraction, the heated interior must be constantly tending to shrink away from the cold external crust.

Of course no actual separation between the crust and interior or nucleus can take place, but there is no doubt that the crust is left unsupported to a certain extent, and it must then behave like an arch with a radius of 4000 miles, and the result is an enormous horizontal or tangential pressure.

This lateral pressure in the earth’s crust is one of the most important and most generally accepted facts in geology, and lies at the bottom of many geological theories. According to what seems to me to be the most probable theory of the origin of continents and ocean-basins, they are broad upward and downward bendings or arches into which the crust is thrown by the tangential pressure. Finally, the strain becomes great enough to crush the crust along those lines where it is weakest. When the crust is thus mashed up by horizontal pressure, a mountain range is formed, the crust becomes enormously thicker, and a weak place becomes a strong one.

During the formation of mountains the stratified rocks, which were originally horizontal, are thrown into folds or arches, and tipped up at all possible angles; they are fractured and faults produced; and by the immense pressure the structure known as slaty cleavage is developed. In fact, a vast amount and variety of structures are produced during the growth of a mountain range.

These great earth-movements are not always perfectly smooth and steady, but they are accompanied by slipping or crushing now and then; and, as a result of the shock thus produced, a swift vibratory movement or jar, which we know as an earthquake, runs through the earth’s crust.

Extensive fissures are also formed, opening down to the regions where the rocks are liquid or plastic, and through these the melted rocks flow up to or toward the surface. That portion which flows out on the surface builds up a volcanic cone, while that which cools and solidifies below the surface, in the fissures, forms dikes. Thus among the igneous or eruptive rocks we have two great classes,—the dike rocks and the volcanic rocks.

It is important to observe that all these subterranean operations—the formation of continents, of mountain-ranges with all their attendant phenomena of folds, faults and cleavage, and every form and phase of earthquake and volcanic activity—depend upon or originate in the interior heat of the earth. And over against the superficial or aqueous agencies, originating in the solar heat and producing the stratified or sedimentary rocks, we set the subterranean or igneous agencies originating in the central heat, and producing the unstratified or eruptive rocks.

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STRUCTURAL GEOLOGY.

In geology, just as in biology, there are two ways of studying structure,—the small way and the large way. In the case of an organism, we may select a single part or organ, and, disregarding its external form and relations to other parts, observe its composition and minute structure, the various forms and arrangements of the cells, etc. This is histology, and it is the complement of that larger method of studying structure which is ordinarily understood by anatomy.

The divisions of structural geology corresponding to histology and anatomy are lithology and petrology. Lithology is an in-door science; we use the microscope largely, and work with hand specimens or thin sections of the rocks, observing the composition and those small structural features which go under the general name of texture.

In petrology, on the other hand, we consider the larger kinds of rock-structure, such as stratification, jointing, folds, faults, cleavage, etc.; and it is essentially an out-door science, since to study it to the best advantage we must have, not hand specimens, but ledges, cliffs, railway-cuttings, gorges, and mountains.

LITHOLOGY.

A rock is any mineral, or mixture of minerals, occurring in masses of considerable size. This distinction of size is the only one that can be made between rocks and minerals, and that is very indefinite. A rock, whether composed of one mineral or several, is always an aggregate; and therefore no single crystal or mineral-grain can properly be called a rock.

Before proceeding to study particularly the various kinds of rocks, a little more preliminary work should be done. As already intimated, the more important characteristics of rocks may be grouped under two general heads,—composition and texture.

Composition of Rocks.

Rocks are properly defined as large masses or aggregates of mineral matter, consisting in some cases of one and in other cases of several mineral species. Hence it is clear that the composition of rocks is of two kinds: chemical and mineralogical; for the various chemical elements are first combined to form minerals, and then the minerals are combined to form rocks.

Of course those minerals and elements which can be described as principal or important rock-constituents must be the common minerals and elements. Now it is a very important and convenient fact that although chemists recognize about sixty-five elementary substances, and these are combined to form nearly one thousand mineral species, yet both the common elements and the common minerals are few in number.