Page:Popular Science Monthly Volume 59.djvu/259

Rh Among the rocks of any range there are youngest strata that mark a date earlier than the most remote at which the uplift may have occurred. Impressed with the magnitude and grandeur of mountains, geologists assigned them an antiquity limited only by the age of their component strata, but through the interpretation of landscape forms evidence is now accumulating to show that existing ranges are, as a rule, comparatively young. One interesting conclusion is that we live at a time near the culmination of an epoch of mountain growth, that mountains are now widely distributed and high as compared with those of many preceding periods, and the earth's activity as thus manifested is not materially less now than formerly within known geologic history. The crustal adjustment which produced existing mountain ranges and expanded continents appears to have culminated just before or very early in the Glacial epoch, and the recognition of this fact was the principal basis for the hypothesis that glaciation was related directly to elevation of the land areas. Chamberlin interprets the relation through the influence of rock-weathering upon the carbon dioxide of the air, and attributes the cold period to the resulting thermal transparency of the atmosphere.

The carbon dioxide abstracted from the air by weathering passes into the aqueous circulation of the globe, one-half of it in combination as monocarbonates, the other half superadded to form bicarbonates. A further step in framing the hypothesis is to follow this second part until it shall be returned to the air, which shall thus be reenriched and may promote a period of mild climate.

The ocean is a great reservoir holding carbon dioxide in combination with various bases as bicarbonate. It contains also many other salts. Assuming that all the solids dissolved in sea-water have been derived from the land at a rate of solution equal to that now determined by analyses of river waters, it is possible to make a curious calculation, which shows that the carbonate of lime now in the sea would have accumulated in 60,000 years, whereas the common salt, chloride of sodium, would have required 166,000,000 years. The common salt is not removed from solution, nor is there reason to suppose that there is any special source from which it is concentrated, but which does not supply lime; it may, therefore, be taken as a standard of comparison, which shows that there is much less lime in the sea than we should expect. The deficit is accounted for by the great beds of limestone deposited from the sea at various periods from the long past to the present.

In the ocean, bicarbonate of lime is dissolved in a proportion less than that which the waters can hold in solution, and, according to the principles of the older chemistry, it is under these conditions a fixed combination, which remains dissolved. Should, however, the