GEOLOGICAL STRUCTIME.-PLUTONIC.
The rocks originally forming the crust of the earth, and resulting from condensation and sublimation, or comparative degrees of heat and cold, are known as plutonic, granitic, or igneous rocks. They form the base of all subsequent formations, and are, in fact, the primary elements from which all our lithological structure is built or derived, since even the volcanic rocks are of the same nature and origin.
We find these primary or igneous and unstratified rocks on our lowest shores and on our highest mountains, while they underlie every other formation over the entire surface of the globe, as the original crust of the earth. From its internal depths, where the creative fires ever struggled for vent, these igneous or volcanic rocks were poured forth in rivers of liquid lava, porphyries, basalt, greenstone, &c., arid deposited through all subsequent ages and formations. Therefore the plutonic and volcanic rocks, though primarily the oldest, have become coextensive and cotem porary with every period of our geological or lithological strata. The igneous mass from which they were derived has been the means, or source, from which all or most of our metamorphic and subsequent sedimentary strata have been formed, either through the agency of volcanic eruption, which vented the molten lava into the ancient seas, or the erosion of floods and storms on the decomposing mountains, which were exposed when the waters were gathered together and the dry land appeared.
It is a question which perhaps would not be difficult of solution, whether or not the primary elements—the vapors from which our globe was formed or condensed—contained the constituents of all subsequent productions. In fact, we cannot doubt the truth of this proposition, since it is evident we have neither lost nor gained in the quantity or constituents of the material elements forming the earth and its atmosphere since the days of creation.
The sixty-five or seventy chemical constituents of the earth or its matter, therefore, not only existed in the nebulous vapor forming the earth, but, of course, they must have existed in the plutonic rocks and the internal fiery mass of the earth, or the atmosphere surrounding it. We thus find in the bowels of the earth all the gases, the primary elements of subse quent chemical production. They formed, directly or indirectly, our coal and our coal-oil, and all the forms of vegetation and life which beautify and animate the works of Nature.