THE CHEMISTRY OF THE ROCKS. 699 



portion of the earth in the making up of its constituents was trans- 

 formed into water-vapor. Hydrogen is found in no other combina- 

 tion 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 inte- 

 rior. 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 trans- 

 formations 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 caus- 

 tic 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 quick- 

 lime 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 appear- 

 ance 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 lime- 

 stones, 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 



