Time and Change

by John Burroughs

The granite is the Adam rock, and through a long line of descent the major part of all the other rocks directly or indirectly may be traced. Thus the granite begot the Algonquin, the Algonquin begot the Cambrian, the Cambrian begot the Silurian, the Silurian begot the Devonian, and so on up through the Carboniferous, the Permian, the Mesozoic rocks, the Tertiary rocks, to the latest Quaternary deposit.

But the curious thing about it all is the enormous progeny from so small a beginning; the rocks seem really to have grown and multiplied like organic beings; the seed of the granite seems to have fertilized the whole world of waters, and in due time they brought forth this huge family of stratified rocks. There stands the Archaean Adam, his head and chest in Canada, his two unequal legs running, one down the Pacific coast, and one down the Atlantic Coast, and from his loins, we are told, all the progeny of rocks and soils that make up the continent have sprung, one generation succeeding another in regular order. His latest offspring is in the South and Southwest, and in the interior. These are the new countries, geologically speaking, as well as humanly speaking.

The great interior sea, epicontinental, the geologists call it, seems to have been fermenting and laboring for untold aeons in building up these parts of the continent. In the older Eastern States we find the sons and grandsons of the old Adam granite; but in the South and West we find his offspring of the twentieth or twenty-fifth generation, and so unlike their forebears; the Permian rocks, for instance, and the Cretaceous rocks, are soft and unenduring, for the most part. The later slates, too, are degenerates, and much of the sandstones have the hearts of prodigals. In the Bad Lands of Arizona I could have cut my way into some of the Eocene formations with my pocket-knife. Apparently the farther away we get from the parent granite, the more easily is the rock eroded. Nearly all the wonderful and beautiful sculpturing of the rocks in the West and Southwest is in rocks of comparatively recent date.

Can we say that all the organic matter of our time is from preexisting organic matter? one organism torn down to build up another? that the beginning of the series was as great as the end? There may have been as much matter in a state of vital organization in Carboniferous or in Cretaceous times as in our own, but there is certainly more now than in early Palaeozoic times. Yet every grain of this matter has existed somewhere in some form for all time. Or we might ask if all the wealth of our day is from preexisting wealth—one fortune pulled down to build up another,—too often the case, it is true,—thus passing the accumulated wealth along from one generation to another. On the contrary, has there not been a steady gain of that we call wealth through the ingenuity and the industry of man directed towards the latent wealth of the earth? In a parallel manner has there been a gain in the bulk of the secondary rocks through the action of the world-building forces directed to the sea, the air, and the preexisting rocks. Had there been no gain, the fact would suggest the ill luck of a man investing his capital in business and turning it over and over, and having no more money at the end than he had in the beginning.

Nothing is in the sedimentary rock that was not at one time in the original granite, or in the primordial seas, or in the primordial atmosphere, or in the heavens above, or in the interior of the earth beneath. We must sweep the heavens, strain the seas, and leach the air, to obtain all this material. Evidently the growth of these rocks has been mainly a chemical process—a chemical organization of preexisting material, as much so as the growth of a plant or a tree or an animal. The color and texture and volume of each formation differ so radically from those of the one immediately before it as to suggest something more than a mere mechanical derivation of one from the other. New factors, new sources, are implied. "The farther we recede from the present time," says Lyell, "and the higher the antiquity of the formations which we examine, the greater are the changes which the sedimentary deposits have undergone." Above all have chemical processes produced changes. This constant passage of the mineral elements of the rocks through the cycle of erosion, sedimentation, and reinduration has exposed them to the action of the air, the light, the sea, and has thus undoubtedly brought about a steady growth in their volume and a constant change in their color and texture. Marl and clay and green sand and salt and gypsum and shale, all have their genesis, all came down to us in some way or in some degree, from the aboriginal crystalline rocks; but what transformations and transmutations they have undergone! They have passed through Nature's laboratory and taken on new forms and characteristics.

"All sediments deposited in the sea," says my geology, "undergo more or less chemical change," and many chemical changes involve notable changes in volume of the mineral matter concerned. It has been estimated that the conversion of granite rock into soil increases its volume eighty-eight per cent, largely as the result of hydration, or the taking up of water in the chemical union. The processes of oxidation and carbonation are also expansive processes. Whether any of this gain in volume is lost in the process of sedimentation and reconsolidation, I do not know. Probably all the elements that water takes from the rocks by solution, it returns to them when the disintegrated parts, in the form of sediment in the sea, is again converted into strata. It is in this cycle of rock disintegration and rock re-formation that the processes of life go on. Without the decay of the rock there could be no life on the land. Water and air are always the go-betweens of the organic and inorganic. After the rains have depleted the rocks of their soluble parts and carried them to the sea, they come back and aid vegetable life to unlock and appropriate other soluble parts, and thus build up the vegetable and, indirectly, the animal world.

That the growth of the continents owes much to the denudation of the sea-bottom, brought about by the tides and the ocean-currents, which were probably much more powerful in early than in late geologic times, and to submarine mineral springs and volcanic eruptions of ashes and mud, admits of little doubt. That it owes much to extra-terrestrial sources—to meteorites and meteoric dust—also admits of little doubt.

It seems reasonable that earlier in the history of the evolution of our solar system there should have been much more meteoric matter drifting through the interplanetary spaces than during the later ages, and that a large amount of this matter should have found its way to the earth, in the form either of solids or of gases. Probably much more material has been contributed by volcanic eruptions than there is any evidence of apparent. The amount of mineral matter held in solution by the primordial seas must have been enormous. The amount of rock laid down in Palaeozoic times is estimated at fifty thousand feet, and of this thirteen thousand feet were limestone; while the amount laid down in Mesozoic times, for aught we know a period quite as long, amounts to eight thousand feet, indicating, it seems to me, that the deposition of sediment went on much more rapidly in early geologic times. We are nearer the beginning of things. All chemical processes in the earth's crust were probably more rapid. Doubtless the rainfall was more, but the land areas must have been less. The greater amount of carbon dioxide in the air during Palaeozoic times would have favored more rapid carbonation. When granite is dissolved by weathering, carbon unites with the potash, the soda, the lime, the magnesia, and the iron, and turns them into carbonates and swells their bulk. The one thing that is passed along from formation to formation unchanged is the quartz sand. Quartz is tough, and the sand we find to-day is practically the same that was dissolved out of the first crystalline rocks.

Take out of the soil and out of the rocks all that they owe to the air,—the oxygen and the carbon,—and how would they dwindle! The limestone rocks would practically disappear.

Probably not less that one fourth of all the sedimentary rocks are limestone, which is of animal origin. How much of the lime of which these rocks were built was leached out of the land-areas, and how much was held in solution by the original sea-water, is of course a question. But all the carbon they hold came out of the air. The waters of the primordial ocean were probably highly charged with mineral matter, with various chlorides and sulphates and carbonates, such as the sulphate of soda, the sulphate of lime, the sulphate of magnesia, the chloride of sodium, and the like. The chloride of sodium, or salt, remains, while most of the other compounds have been precipitated through the agency of minute forms of life, and now form parts of the soil and of the stratified rocks beneath it.