I.—The Fundamental Problems of Petrogenesis, or the Origin of the Igneous Rocks

Abstract

The second fundamental problem of petrogenesis is the following question:—By what processes have there been derived, from the original magma or magmas, all other magmas known as igneous rocks? It is now almost universally admitted that many igneous rocks are genetically connected and produced from an original magma by differentiation, and that in many cases differentiation may be considered as liquation or separating into secondary magmas. But is differentiation (Spaltungen) alone sufficient to explain the formation of all igneous rocks? And by what is a magma stimulated to differentiation? These are two questions which must be elucidated. It must be first of all emphasized that not every magma is subject to differentiation. My standpoint is that the principal factors producing and regulating differentiation are on one side the process of crystallization (differentiation by crystallization, Krystallizations-differenzierung), and on the other the chemical composition of the magma and its tendency to the formation of eutectics (magmatic differentiation).