CHAPTER III.

OF THE CHANGES TO WHICH THE CRUST OF THE EARTH HAS BEEN SUBJECTED.

SECTION I.--CHANGES WHICH HAVE TAKEN PLACE AT GREAT DEPTHS BELOW THE SURFACE.

The lowest change of winch we can gain any information is the formation of granite. It will be shown hereafter that it has been in a melted state, and that it has taken its present form on cooling. But whether any considerable portions of the granitic masses, or of the melted masses now below the surface, have resulted from the fusion of stratified rocks, we have not the means of determining. It is, however, not improbable, that in the changes of level to which the crust of the earth has been subjected, the stratified rocks may have gone down so far as to become melted. At the same time, the melted rock which is thrown to the surface by volcanoes is subjected to the various destroying agencies by which it becomes sedimentary matter, to be deposited as mechanical strata. Thus, as the igneous rocks from below are brought up to furnish materials for mechanical strata, there must be an equal amount of depression of the mechanical strata towards the seats of igneous action. And if this change takes place more rapidly than the thickness of crust increases, then portions of the sedimentary rocks must be undergoing fusion.

Next above the granite an immense thickness of rock occurs, which exhibits, from its stratification and from the water-worn fragments which it contains, distinct evidence of its mechanical origin. And yet it is very different from the later mechanical formations. It is more highly crystalline; it has, to a great extent, assumed a cleavage distinct from the planes of stratification, and chemical affinity has been so far active as to produce new combinations, and give to them their peculiar crystalline form, as in the case of garnets, iron pyrites, &c. These strata also differ from those above them in containing no organic remains. It is not certain that organic life existed on the earth at the time when these rocks were deposited. Either it did not, or the evidence of it in the strata of that period has been obliterated. The changes have at least been sufficient to justify their being characterized as metamorphic rocks.