THE EVOLUTION OF IGNEOUS ROCKS. 



THE labours of many workers during the thirty years 

 which have elapsed since the revival of petrological 

 study are already bearing fruit, and geologists in general 

 are not slow to perceive the value of the new data already 

 at their disposal with reference to some of the more difficult 

 problems of their science. Indeed, it is not too much to 

 say that from the purely descriptive study which has been 

 content to describe itself as petrography, there is already 

 emerging a philosophy of petrology, which, we may expect, 

 will soon be able to deal successfully with very broad 

 questions. Of these questions none is of greater interest 

 or more fundamental importance than that of the origin and 

 mutual relations of igneous rocks. 



The day is past when a writer was content to describe 

 a rock as consisting of certain specified minerals, and 

 thought, for instance, that he sufficiently accounted for a 

 high silica-percentage in an analysis by stating that the 

 rock contained free quartz! It is recognised that, in a rock 

 formed from igneous fusion, the chemical composition must 

 be in general the prime datum ; so that it is not the minerals 

 that form the rock, but the rock (or rather the rock-magma) 

 that forms the minerals. Petrologists have thus been led 

 to discuss the physics and chemistry of the molten magmas, 

 by the consolidation of which igneous rocks have been 

 formed, and, in particular, to inquire how such diversity 

 could arise in the composition of these magmas as is 

 proved by the great variety observed among the result- 

 ing rocks. If we grant, as is conceivable and indeed 

 probable, that a rock can be formed from a magma of 

 different composition from its own, the conditions under 

 which this might take place open up a new branch of the 

 problem. 



It has long been known that igneous rocks of widely 

 different chemical (and in part mineralogical) composition 

 often occur in close association, and many facts seem to 



