268 PKOCEEDINGS OP SECTION C. 



Yogt's observations were found to be in accord with modern theories 

 of solutions, as Bunsen foretold in 1861, when he affirmed his belief 

 that rock magmas are solutions of silicate compounds liquid at high 

 temperatures. Vogt has called the attention of petrologists to these 

 modern theories as developed by Arrhenius, van 't Hoff, Ostwald, 

 Gibbs, Meyerhoft'er, Roozeboom, and others. He has also made definite 

 application of them to some of the phenomena • and relationships 

 mentioned. His publications have extended widely the horizon of 

 modern petrology, which by the assumption of these broader, deeper 

 phases of the study of igneous rocks, and of similar problems affecting 

 nietamorphic rocks, has passed beyond the narrower boundary of 

 petrography, strictly so called. 



The evolution of chemistry from a state of pure empiricism to one 

 of comparatively logical sequence, largely through the assistance of its 

 helpmate physics, has placed before us a collection of co-ordinated 

 laws, which, while incomplete, or subject to numerous exceptions, 

 furnishes us with means to postulate reactions between the constituent 

 elements of rock magmas with reasonable assurance of correctness, or 

 to explain the formation of mineral compounds hitherto in a measure 

 enigmatical. Much remains to be more firmly established, both as to 

 the chemical character of the elements and their compounds, and also 

 with regard to theories relating to their reactions, even to the very 

 nature of their existence in some instances. The silicate compounds 

 constituting igneous rocks remain largely uninvestigated, so far as con- 

 cerris their synthesis and reactions in mutual solution. And the physical 

 study of solutions and of their transitions to the solidified components, 

 especially the more complex mixtures, is far from completed. The 

 present is a period of transition in the development of petrology, as 

 were also times past. But the changes taking place at this time 

 appear to be so many and so fundamental that it may well be asked 

 whether the older methods of approach to the study of igneous rocks 

 should not be replaced by others more in accord with present con- 

 ditions of knowledge of chemistry, physics, and of the rocks them- 

 selves. The older method, in the nature of things, was, and is largely 

 at the present day, objective, and the expressions of relationships or 

 laws empirical. 



It would seem more reasonable to begin a systematic study of 

 igneous rocks with a consideration of the most fundamental charac- 

 teristics of the magmas from which tliey have solidified; of their 

 constitutents, together with their probable chemical reactions and the 

 resulting mineral compounds ; of the manner in which these may 

 separate from a silicate solution, or rock magma ; of the shapes they 

 are likely to assume upon crystallisation and the consequent texture 

 of the rock". Processes of molecular separation of magmas lead to the 

 discussion of the differentiation of magma into chemically unlike parts, 

 and the resulting different varieties of igneous rocks, together with 

 their eruption and solidification as geological bodies of various kinds. 



Assuming a certain elementaiy acquaintance with rocks on the 

 part of the student, which is acquired in courses on general geology, 

 the systematic treatment of the subject should begin by calling 

 attention to the chemical composition of unaltered igneous rock as 

 shoMTi by analyses published in many descriptions of rocks, but most 

 conveniently found in comprehensive collections in bulletins of the 



