On the evolution and classification of Igneous rocks

Abstract

With your permission I will to-night take the opportunity of making some remarks on a branch of Petrology in which I am specially interested, and which I have studied more or less independently for several years. The subject, or petrological department, to which I refer is that of Igneous, or Original Rocks; and as the result of my labour I will lay before you what I consider to be practically a new classification of these rocks, which I hope may prove to be a rational one. Before, however, proceeding to discuss the arrangement of the eruptive masses of the earth’s crust, I will claim your indulgence while I lay before you as a kind of introduction a few facts relating to their formation and structures. By the term ‶Igneous,″ I, of course, refer to a rock which has solidified from a state of fusion by heat. ‶Igneous Fusion″ is in no way analogous to that which has been undergone by artificially produced rocks, such as slags and glasses. These have consolidated from a condition of dry fusion, whereas the melting of igneous rocks has always taken place in the presence of water, and their subsequent devitrification has always been accomplished by the active aid of that liquid, and as the result of combination with it. That this is perfectly true can be easily learned from a simple analytical examination of any igneous rock. If you take a piece of granite, pound it to powder, and after thoroughly drying it in