Part 2
We will now observe the process of deposition a little more closely. Each of these two bottles contains the same amount of fine yellow clay, but in one the water is fresh, and in the other it is salt. At the beginning of the lesson, as you may have observed, I brought the clay in both bottles into suspension by violent agitation, and since then they have remained undisturbed. The main point is that the salt water has become quite clear, while the fresh water is still distinctly turbid, showing that the salt favors the rapid deposition of the clay. At the second lecture, a week later, these two bottles, yet undisturbed, were exhibited, and the fresh water seen to be still sensibly turbid. The fact is, the clay is not held in suspension wholly by the _motion_ of the water; but, just as in the case of dust in the atmosphere, a small portion of the medium is condensed around or adheres to each solid particle, _i.e._, each clay particle in our experiment has an atmosphere of water which moves with it and buoys it up. Now the effect of the salt is to diminish the adhesion of the water to the particles, _i.e._, to diminish their atmospheres, and consequently their buoyancy. The diminished adhesion of the salt water is well shown by the smaller drops which it forms on a glass rod.
The geological importance of this principle is very great; for it is undoubtedly largely to the saltness of the sea that we owe its transparency, and the fact that the fine, clayey sediment from the land, like the coarse, is deposited near the shore.
This bottle of fresh water contains some fine gravel, coarse sand, fine sand, and clay. By agitating the water, all this material is brought into suspension. Now, suddenly placing the bottle in a state of rest, we observe that the gravel falls to the bottom almost instantly, followed quickly by the coarse sand, and very soon afterward by the fine sand; and then there appears to be a pause, the fine particles of clay all remain in suspension; but finally, when the water is quite motionless, they begin to settle; they fall very slowly, however, and the water will not be clear for hours.
This is a very instructive experiment. We learn from it:
First, that the power of the water to hold particles in suspension is inversely proportional to the size of the particles;
Second, that all materials deposited in water are assorted according to size;
Third, and this is one of the most important facts in geology, all water-deposited sediments are arranged in horizontal layers, _i.e._, are stratified. And we have now traced to its conclusion, though very briefly, the process of the formation of one great division of _stratified_ rocks,—the _mechanically-formed_ or _fragmental_ rocks. These are so called because the clay, sand, and gravel are, in every instance, fragments of pre-existing rocks; and because the formation, transportation, and especially the _deposition_ of these fragments, are the work chiefly or entirely of mechanical forces.
CHEMICAL DEPOSITION.—It is a well-known fact that the sea holds in solution vast amounts of common salt as well as many other substances; and analyses of river-waters show that dissolved minerals derived from the chemical decomposition of the rocks of the land are being constantly carried into the sea.
Portions of the sea which are cut off from the main body, and which are gradually drying up, like the Great Salt Lake, Dead Sea, and Caspian Sea, become saturated solutions of the various dissolved minerals, and these are slowly deposited. This process is very nicely illustrated along our shores in summer, where, during storms, salt-water spray is thrown above the reach of the tides, and, collecting in hollows in the rocks, gradually dries up, leaving behind a crust of salt.
When the sea lays down matter which it held in _suspension_, we call the process _mechanical_ deposition, and the result is _mechanically_-formed rocks.
But when it lays down matter which it held in _solution_, we call the process _chemical_ deposition, and the result is _chemically_-formed rocks.
The principal substances which the sea deposits chemically are common salt, forming beds of rock-salt; sulphate of calcium, forming beds of gypsum; carbonate of calcium, forming beds of limestone; and the double carbonate of calcium and magnesium, forming beds of dolomite.
Inorganic deposition, like inorganic erosion, is both chemical and mechanical.
2. _Animals and Plants, or Organic Agencies._
We turn now to the consideration of the _organic_ agencies. And I will merely allude in passing to the vast importance of the fossil organic remains found in the stratified rocks as marks by which to determine the relative ages of the formations.
As regards the _destruction_ of rocks—_erosion_—plants and animals are almost powerless; but in the role of _rock-makers_ they play a very important part, being very efficient agents of _deposition_.
FORMATION OF COALS AND BITUMENS.—Specimen No. 8 is an example of peat from the vicinity of Boston; but just as good specimens may be obtained in thousands of places in this and other States.
The general physical conditions under which peat is formed are familiar facts. We require simply low, level land, covered with a thin sheet of water and abundant vegetation; in other words, a marsh or swamp. If plants decay on the dry land, the decomposition is complete; they are burned up by the oxygen of the air to _carbon dioxide_ and _water_ just as surely as if they had been thrown into a furnace, though less rapidly, and nothing is returned to the soil but what had been taken from it by the plants during their growth. But if the plants decay under water, as in a peat-marsh or bog, the decay is incomplete, and most of the carbon of the wood is left behind. Now, if this incomplete combustion of vegetable tissues takes place in a charcoal-pit, where the wood is out of contact with air from being covered with earth, we call the carbonaceous product charcoal; but if under the water of a marsh, in Nature’s laboratory, we call the product peat. Peat is simply a natural charcoal; and, just as in ordinary charcoal, its vegetable origin is always perfectly evident. But when the deposit becomes thicker, and especially when it is buried under thick formations of other rocks, like sand and clay, the great pressure consolidates the peat; it becomes gradually more mineralized and shining, shows the vegetable tissues less distinctly, becomes more nearly pure carbon, and we call it in succession lignite, bituminous coal, and anthracite.
This is, briefly, the way in which all varieties of coal, as well as the more solid kinds of bitumen, like asphaltum, are formed. But the lighter forms of bitumen, such as petroleum and naphtha, are derived mainly, if not entirely, from the partial decomposition of animal tissues. These, it is well known, decay much more readily than vegetable tissues; and the water of an ordinary marsh or lake contains sufficient oxygen for their complete and rapid decomposition. In the deeper parts of the ocean, however, the conditions are very different, for recent researches have shown, contrary to the old idea, that the deep sea holds an abundant fauna. All grades of animal life, from the highest to the lowest, have need of a constant supply of oxygen. On the land vegetation is constantly returning to the air the oxygen consumed by animals, but in the abysses of the ocean vegetable life is scarce or wanting; and hence it must result that over these greater than continental areas countless myriads of animals are living habitually on short rations of oxygen, and in water well charged with carbon dioxide, the product of animal respiration. As a consequence, when these animals die their tissues do not find the oxygen essential for their perfect decomposition, and in the course of time become buried, in a half-decayed state, in the ever-increasing sediments of the ocean-floor.
It is important to observe that an abundance of organic matter decaying under water is not the only condition essential to the formation of beds of coal and bitumen; for this condition is realized in the luxuriant growth of sea-weeds fringing the coast in every quarter of the globe; and yet coals and bitumens are rarely of sea-shore origin. These organic products, even under the most favorable circumstances, accumulate with extreme slowness; far more slowly, as a rule, than the ordinary mechanical sediments, like sand and clay, with which they are mixed, and in which they are often completely lost. Consequently, although the deposition of the carbonized remains of plants and animals is taking place in nearly all seas, lakes, and marshes, it is only in those places where there is little or no mechanical sediment that they can predominate so as to build up beds pure enough to be called coal or bitumen. In all other cases we get merely more or less carbonaceous sand or clay. Now these especially favorable localities will manifestly not be often found along the seashore, where we have strewn the sand and clay brought down by rivers or washed off the land directly by the ever-active surf; but they must exist in the central portions of the ocean, where there is almost no mechanical sediment and yet an abundance of life, and in swamps and marshes, where there is scarcely sufficient water to cover the vegetation, and no waves or currents to wash down the soil from the surrounding hills.
FORMATION OF IRON-ORES.—The iron-ores are another class of rocks which are formed only through the agency of organic matter. Iron is an abundant and wide-spread element in the earth’s crust, and, but for the intervention of life, we might say that, while there is iron everywhere, there is not much of it in any one place, since it is originally very thinly diffused. All rocks and soils contain iron, but it is mainly in the form of the peroxide, in which state it is entirely insoluble, and hence cannot be soaked out of the soil by the rain-water and concentrated by the evaporation of the water at lower levels in ponds and marshes, as a soluble substance like salt would be. If carried off with the sand and clay, by the mechanical action of water, it remains uniformly mixed with them, and there is no tendency to its separation and concentration so as to form a true iron-ore.
But what water cannot do alone is accomplished very readily when the water is aided by decaying organic matter, which is always hungry for oxygen, being, in the language of the chemist, a powerful reducing agent. The soil, in most places, has a superficial stratum of vegetable mould or half-decayed vegetation. The rainwater percolates through this and dissolves more or less of the organic matter, which is thus carried down into the sand and clay beneath and brought in contact with the ferric oxide, from which it takes a certain proportion of oxygen, reducing the ferric to the ferrous oxide. At the same time the vegetation is burned up by the oxygen thus obtained, forming carbon dioxide, which immediately combines with the ferrous oxide, forming carbonate of iron, which, being soluble under these conditions, is carried along by the water as it gradually finds its way by subterranean drainage to the bottom of the valley and emerges in a swamp or marsh.
Here one of two things will happen: If the marsh contains little or no decaying vegetation, then as soon as the ferrous carbonate brought down from the hills is exposed to the air it is decomposed, the carbon dioxide escapes, and the iron, taking on oxygen from the air, returns to its original ferric condition; and being then quite insoluble, it is deposited as a loose, porous, earthy mass, commonly known as bog-iron-ore, which becomes gradually more solid and finally even crystalline through the subsequent action of heat and pressure. When first deposited, the ferric oxide is combined with water or hydrated, and is then known as limonite (specimen No. 12); at a later period the water is expelled, and we call the ore hematite (specimen No. 13); and at a still later age it loses part of its oxygen, becomes magnetic and more crystalline, and is then known as magnetite (specimen No. 14). Thus it is seen that the iron-ores, as we pass from bog-limonite to magnetite, form a natural series similar to and parallel with that afforded by the coals as we pass from peat to graphite.
If the drainage from the hills is into a marsh containing an abundance of decaying vegetation, _i.e._, if peat is forming there, the ferrous carbonate, in the presence of the more greedy organic matter, will be unable to obtain oxygen from the air; and as the evaporation of the water goes on, it will sooner or later become saturated with this salt, and the latter will be deposited. Here we find an explanation of a fact often observed by geologists, viz., that the carbonate iron-ores are usually associated with beds of coal.
The formation of the iron-ores, like that of the coals and bitumens, is a slow process; and the ores, like the coals, etc., will be pure only where there is a complete absence of mechanical sediment, a condition that is realized most nearly in marshes.
FORMATION OF LIMESTONE, DIATOMACEOUS EARTH, ETC.—Marine animals take from the sea-water certain mineral substances, especially silica and carbonate of calcium, to form their skeletons. Silica is used only by the lowest organisms, such as Radiolaria, Sponges, and the minute unicellular plants, Diatoms. The principal animals secreting carbonate of calcium are Corals and Mollusks. These hard parts of the organisms remain undissolved after death; and over portions of the ocean-floor where there is but little of other kinds of sediment they form the main part of the deposits, and in the course of ages build up very extensive formations which we call diatomaceous earth or tripolite, if the organisms are siliceous, or limestone if they are calcareous. A very satisfactory account of the formation of limestone on a stupendous scale by the polyps in coral reefs and islands is contained in No. IV. of this series of guides.
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.
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.