VII.—Classification of Igneous Rocks according to their Chemical Composition.

Extract

In my previous paper (see Geological Magazine, March 1906, p. 131) I explained a method of separating igneous rocks into sixteen groups of chemically similar individual rocks. A more minute subdivision appears, however, desirable for the special purpose of identifying and describing particular rocks. I have therefore devised the following further classification with four times as many groups, and I trust that the proximity thus attained between the individual rocks of each group will amply meet all requirements.