Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Probable fluctuations of atmospheric composition.—With this general sketch of the interplay of the atmospheric elements under organic influence, we are prepared for the further conception that if one or another of these actions was relatively more vigorous than usual for a period, it would bring about a variation in the proportions of the atmospheric constituents. If, for example, vegetation flourished luxuriantly for a long period, but was measurably protected from the organisms that preyed upon it and from inorganic decomposition, as by falling into water or by prompt burial under sediment, the atmosphere might be growing richer in oxygen. If, on the other hand, vegetation were being relatively reduced, as perhaps it is being reduced now by man, and if previous organic products were being reoxidized at an unusual rate, as they are now in the burning of timber, coal, natural oil and gas, the carbon dioxide of the atmosphere might be relatively increasing, while the oxygen might be relatively diminishing. The possible fluctuations of the atmosphere as the result of organic action are, therefore, matters of vital importance, and invite attention in the historical study of the earth and in the outlook into its future.

The climatic effects of organic action.—Interest does not, however, rest at this point. The researches of physicists have made it probable, if they have not altogether demonstrated, that the composition of the atmosphere has much to do with the climatic conditions at the surface of the earth. The atmosphere blankets the earth and equalizes its temperature. Acting as a screen, it subdues in some measure the intensity of the sun’s rays by day, while it retards the radiation of the earth’s acquired heat at night. This is in some measure the function of all the constituents of the atmosphere, but by no means of all equally. The oxygen and nitrogen are relatively diathermous, letting the sun’s rays pass in freely, and the earth’s rays pass out freely; but carbon dioxide and the vapor of water are much less diathermous, particularly to rays of low intensity, such as are thrown out by non-luminous bodies like the earth. It follows that while the solar rays come in rather freely and heat the surface of the earth, the dark rays which the earth radiates back are measurably arrested by the carbon dioxide and vapor of water, and serve to keep the air warm. The influence of the vapor of water is vividly shown in the different degrees to which cooling takes place at night in a dry and in a moist atmosphere, respectively, where other conditions are the same. Ice is said to form at night in desert regions where the air is extremely dry, even within the tropics, while in humid regions of the same latitude and altitude oppressively hot nights are common. The influence of the carbon dioxide is not thus familiarly demonstrated, since its amount varies but slightly in different localities, but physical experiment indicates that it has a similar function.

If the amount of carbon dioxide in the atmosphere varies from age to age, the climate of the earth must apparently vary accordingly, and on this is built one of the hypotheses of climatic variation subsequently to be considered. We shall find that there have been great changes in the climate of the earth during its history. There is good evidence of former glaciation, not only in the northern United States and in England, Germany, and central Russia, but in India, Australia, and South Africa. At other times, figs and magnolias grew in Greenland and Spitzbergen, and corals flourished in the Arctic seas. There is good evidence of arid periods where humidity now prevails, and of humid periods where aridity now prevails. It is not assumed that the influence of organic action on the atmosphere has been the sole, or perhaps even the main, cause of these great climatic changes, but it is believed that it has been an important contributing factor. It is even possible that the climate of the future is much dependent on the agency of man, as implied above, however little ground there may be to suppose that he will, with altruistic purpose, control his action with a view to its bearing on the generations that may live tens of thousands of years hence.

(2) Aid and hindrance to inorganic action.

The promotion of disintegration.—While the influence of organic action on the lithosphere is quite superficial, and far less radical than that on the atmosphere, it is still important. Plants promote both disintegration and disaggregation under certain conditions, and hinder them under others, as already set forth. Chemical action of a decomposing and solvent nature takes place in connection with the roots of plants, while their growth sometimes rends rocks into whose crevices they have insinuated themselves. The acids and other products of organic growth and of organic decomposition attack some of the constituents of the rocks and contribute to their solution and disintegration. On the other hand, organic matter entrapped in the sediments, and so introduced into the strata at various depths, often acts as a reducing agency, causing the deposit of substances carried in solution in the underground-waters. Ores are sometimes thus formed, as explained in the discussion of ore-deposits ([p. 476]). Organic action on the whole promotes solution and disintegration at the surface, and prepares the way for deposition below.

Protection against erosion.—Another important function of vegetation is the protection of the land surface against erosion, as already noted in the discussion of erosion. A mantle of grass, especially if it forms a turf, or a carpet of leaves protected by bush and forest, greatly retards surface wash. It does this not only because it directly covers the soil, but because it holds back the run-off and tends to prevent those violent floods which give to erosion its greatest intensity. There is a marked difference between the erosive work which a given amount of water will do if, in the one case, it runs off gradually, and in the other, precipitately. By way of offset, it is to be noted that the disintegrating action of vegetation prepares the rock material for easy erosion, and to this extent helps in its removal by the drainage; but on the average this is greatly overbalanced by the protection afforded by the vegetal covering, though this is not true in every instance.

The influence of land vegetation on the character of the sediments.—The presence or absence of a vegetal covering influences the kind of deposit which is derived from the land, particularly if the surface be occupied by crystalline rocks. If the surface be well clothed with vegetation, the crystals of the complex silicates, such as the feldspars, micas, and ferromagnesian minerals, are usually disintegrated into clayey products before they are removed, so that, when borne away and deposited, the result is common shale. Concurrently, the relatively undecomposable quartz-grains are rounded into sand, and deposited as common quartzose sandstone, while the calcareous material is borne away in solution and deposited as limestone. But if the surface be bare of vegetation, the crystalline rocks are usually disaggregated before they are decomposed, for destructive action works best at the junctions of crystals, and along cleavage lines, and hence the crystals are usually separated from one another before they are fully decomposed. In the absence of a covering to hold them in place until they are decomposed, they are apt to be washed away, and the resulting deposit consists in considerable part of grains of feldspar, mica, hornblende, and other minerals, which do not usually occur in well-decomposed sediments. The deposits are, therefore, of the nature of arkose, if the original rocks are granitic, or of the nature of wacke, as the term is used in this book, if they are of the basic type. On this is based the inference that a vegetal covering of the land extended as far back in the history of the earth as clay shales, quartzose sandstones, and limestones form the prevailing sediments.

(3) Distinctive deposits.

Organic rocks.—In the chapter on the origin and descent of rocks, a group of rocks formed directly from organic matter is recognized and described. The chief of these are peat, lignite, bituminous coal, anthracite, and graphite. It is the belief of many geologists that natural gases, oils, and asphalts are also mainly derived from animal and vegetal remains. An alternative view, advocated by Mendelejeff and Moissan, assigns the oils, gases, etc., in part at least, to deep-seated carbides to which water has gained access and developed hydrocarbons, after the analogy of acetylene.[289] Whatever may be the truth relative to inorganic action, it is clear from geological conditions that some of the natural gases and oils are organic products. Besides the more common organic deposits, there is a long list of minor products, among which are amber, copalite, paraffine, ozocerite, camphene, etc. Guano and coprolites represent the excrementitious class.

Inorganic rocks due to life.—Besides these deposits of organic matter, or of its decomposition products, there is a large class formed from the inorganic matter that served auxiliary functions in the economy of life, such as shells, skeletons, etc. For the greater part these are composed of calcium carbonate, and give rise to limestones, marls, chalk, etc. Not a few, however, are silicious, and give rise to flints, cherts, and silicious earths. Some are formed of calcium phosphate, and a few of other inorganic material. The deposits formed in these ways have been defined in the chapter on rocks.