The Formation of Igneous Rocks

Abstract

The solid matter of which igneous rocks are composed consists either of non-crystalline material (glass), of crystals, or of both crystals and glass. Magmas become igneous rocks by solidification without crystallisation (formation of glass), or by crystallisation, with the loss of much of their volatile material. The difference between crystalline and glassy matter is analogous to the difference between a disciplined battalion and a scattered mob. The molecules or ultimate particles of a crystal are arranged in a definite manner; they are, as it were, neatly packed. In a glass, however, the molecules have settled down into a stable arrangement but without any recognisable pattern. A glass must be regarded as an amorphous solid, not as a super-cooled liquid possessing high rigidity.