Chapter 6. Igneous rock classification and differentiation
Published:June 01, 2015
Extract from beginning of chapter:
THE PROBLEM OF ROCK CLASSIFICATION
The most debatable subject in petrology is that of rock classification, for it is not a matter of facts, but a question of their arrangement and systematic presentation. It is a matter of opinion as to the relative importance of various characters and relationships of rocks, and depends upon the objects or purposes for which any particular arrangement of petrographical facts is designed. The facts to be taken into consideration increase in number and complexity with the advancement of petrological knowledge, and the purposes of classifications vary according to the uses to which a classification is to be put, or according to what its users expect to get out of it. It is to be expected then that, in so new a branch of science as petrography, the classification of igneous rocks must be an unsettled and constantly shifting problem, not only because of improvements in petrological conceptions but on account of changing purposes on the part of petrologists.
Starting with a classification that had been handed to me ex cathedra, I was not greatly concerned at first as to its construction and was occupied with problems of mineral identification; the composition, texture, and variability of igneous rocks; the existence of series; and the absence of distinctions referable to geological age. One of my first arguments on the subject of rock classification seems to have been with James D. Dana regarding quartz as a mineralogical factor in rock definitions. In a letter from him, Dana