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Page:Popular Science Monthly Volume 4.djvu/719

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THE CHEMISTRY OF THE ROCKS.

portion of the earth in the making up of its constituents was transformed into water-vapor. Hydrogen is found in no other combination that cannot be traced directly or indirectly to the decomposition of water.

The aqueous vapor being thus formed, and lying in the upper and cooler regions of the air, it began after a time to condense and fall toward the earth. Meeting with warmer strata as it descended, it was soon evaporated and sent up with a load of heat that was set free again by a recondensation. Then another and perhaps lower descent for another charge of heat. Thus, on the outskirts of the air, water-vapor was cooperating in the work of the heavier vapors of the interior. It was the great fire-carrier of the globe during all the time of the contraction and consolidation of the lower elements. When every thing else that was condensable had turned to dust and ashes, and fallen to the earth, at last the waters reached the parched and scorious surface, and commenced that grand series of aqueous transformations which made a new earth for the indwelling of life.

In the first place, it was necessary that the upper crust should be hydrated, precisely as lime is slaked by pouring water on it. The material which had been last deposited was in reality this same caustic lime. In its lower deposits it was gradually intermixed with the silicious compounds, until these formed the masses which are now the unstratified granitic rocks. As every one knows, the slaking of quicklime absorbs a large quantity of water, which is incorporated into the solid, and great heat is evolved with enlargement of bulk. The pure silicious rocks do not take up water in this way, being what is termed anhydrous. All the rock-materials, then, that lie above the granite must, at some time, have undergone this hydrating, reheating, and swelling process. We accordingly find that all those strata which have remained in their original position, such as the gneiss, the mica schists, the clay-slates, and the primary limestones, have the appearance of having been subjected to great heat and pressure, after having been acted upon by water and steam. In some instances they have been partially melted, in others strangely contorted, and in others partly dissolved. Under certain circumstances, hot water and steam will dissolve small portions of silica, and, if charged with carbonic-acid gas, will dissolve lime quite freely.

The rainfalls of the primeval ages must have been fully saturated with this oxide of carbon, which has played such an important part in the making up of the strata. In this form it carbonated all the limestones, carried all the building-materials to the shell and coral land-makers, and furnished the supplies for the immense magazines of the hydro-carbons. And, after all this, there was enough carbonic-acid gas left in the air for the enormous vegetation of the coal-beds. But it was necessary that the carbon of this gas should be laid away in the earth in some form, either burnt or unburnt, before air-breathing