THE EVOLUTION OF PETROLOGICAL IDEAS. '^03 



The type of inag-matic differentiation conceived by Durocher may 

 be ilkistrated 1)}^ a very simple experiment. Place some phenol and 

 water in a Florence flask; two immiscible conjugate solutions will be 

 formed — a solution of water in phenol at the bottom and a solution 

 of phenol in water at the top. Now heat the mixture to 69° C, and a 

 perfectly homogeneous solution will be produced. On cooling, this 

 will again break up into two. Clouds are tirst formed in the cooler 

 portions of the liquid, and after the coalescence of the minute drops, 

 gravity is able to effect a perfect separation of the two solutions. 



Do silicate solutions behave in the same way '( Backstrom has 

 recently argued that they do; but until the fact has ])een definitely 

 established by experiment, there will always remain a certain element 

 of doul)t. The sharj) separation of basalt and granophyre, which 

 is so striking a feature of the Brito-Icelandic [)rovince, suggests 

 that the two magmas represented by these rocks may separate in the 

 manner just descri])ed. But the great viscosity of fused granophyre 

 at atm()s})heric pressure and easil}^ accessible temperatures would 

 prol)ably prevent the attainment of an}^ decisive result. 



Clarence King maintained that local lakes of fusion were formed 

 hy relief of pressure, and that differentiation took place partly l)y 

 liquation in Durocher's sense, and i)artly l)y the rise or fall of crystals. 



The physico-chemical speculations, which played so important a })art 

 in the science of rocks during the nuddle of the century, were neglected 

 for a time, in consequence of the opening- up of a new field of observa- 

 tion l)y the introduction of the microscope; but of late years we have 

 returned to these specidations with renewed vigor, and with a wealth 

 of facts at our disposal which the earlier theorists would have envied. 



The mineralogical composition and microscopic structure of all 

 kinds of igneous rocks have been determined, reliable chemical 

 anal3'ses have been made, and the problem of th(^ origin of petro- 

 graphical species has resolved itself into the (juestion of the evolution 

 of the magmas. Especially noteworthy is the stinnilus given to the 

 chemical side of petrology by the magnificent work of the Ignited 

 States Geological Survey. We have now some four or five new and 

 original classifications of igneous rocks largely based on the analyses 

 of Clarke, Hiilel^rand. and their assistants, and the cry is, "Still thej' 

 come!" But the authors of these analyses have hitherto refrained- — 

 perhaps wisel}" — from attempting- any general classification of rocks 

 from a chemical point of view. The number of constituents is so 

 large that there is no reason, so far as I can see, why eveiy petrog- 

 rapher should not have his own classification and his own method of 

 graphical representation. 



The idea that petrogra])hical sju'cies ha\ e originated by diffcrtMitia- 

 tion from homogeneous magmas, and possibly in the first instance 

 from some one primordial magma, has been greatl}' developed during 

 the last decade of the century, especially by American and Norwegian 



