Page:Popular Science Monthly Volume 4.djvu/544

526, leaving behind heaps of kaolin clays, such as now form, for the same reason, from the decomposition of granite. This reaction is one peculiar to dry land, and would therefore be subsequent in time to the changes already mentioned. Now, the potassium and sodium carbonates, when brought into contact with calcium chlorides, change their composition, and there result calcium carbonates and sodium and potassium chlorides. These carbonates, being insoluble, will be precipitated to the bottom, and thus will be formed the primitive travertines and limestones, while the sodium chlorides remain in solution to this day, save what has been converted into beds of rock-salt.

With the removal of the bulk of the acids and possible volatile compounds from the atmosphere, only carbonic acid would remain to render it impure at the close of the era of chemical changes. In later periods this part of the atmosphere has also been removed. The world is not yet ready for life, as there must be further chemical and mechanical changes.

—The next era brings into play a phase of action destined to be the chief agent of change in the world—the erosion of existing ledges to form new rocks. The era opens with a continuation of the atmospheric decompositions, whereby we find silica and alumina remaining in irregular heaps of sand and clay, and the accumulation of calcareous deposits beneath the ocean.

The formation of thick deposits of inorganic limestone is extremely interesting. Scientists have been wont to ignore altogether the existence of any deposit of this character, since microscopic researches into the structure of many of the calcareous masses exposed at the surface indicate an organic origin. So many shells and coral fragments aid in building up fossiliferous limestone that its mode of growth is very clear. But, after one has spent months in searching vainly for traces of organisms among the marble layers of Western Vermont, or the auroral limestones of Eastern Pennsylvania, he is tempted to suspect that some of the Silurian limestones even were chemical deposits, though wanting the concentric structure of stalagmite and travertine. But, barring these, and the calcareous dikes in the Laurentian of Northern New York, and in the Silurian beds of Northern Vermont, all the phenomena are best explained by the presence of an inorganic limestone before the origin of life. Whence came the materials for the stony habitations of marine animals? There must have existed great masses of the crude material, stored up in the rocks and in the waters of the sea, to provide with coverings all the testacea of every age, and to furnish the thousands of feet thickness of the Eozoic, Paleozoic, and Mesozoic limestones. This primitive source of supply is now concealed, but much of its material has been used over and over again.

We have suggested how three of the principal rock-materials have been formed—the quartz, clay, and limestone. We have them yet as rude piles of rubbish, neither arranged in layers nor possessing any