IXXX PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9OI, 



69° C, and a perfectly homogeneous solution will be produced. On 

 cooling, this will again break up into two. Clouds are first 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 been definitely 

 established by experiment, there will always remain a certain 

 element of doubt. The sharp separation of basalt and granophyre, 

 which is so striking a feature of the Brito-Icelandic province, 

 suggests that the two magmas represented by these rocks may 

 separate in the manner just described. But the great viscosity of 

 fused granophyre at atmospheric pressure and easily accessible 

 temperatures would probably prevent the attainment of any decisive 

 result. 



Clarence King maintained that local lakes of fusion were formed 

 by relief of pressure, and that differentiation took place partly by 

 liquation in Durocher's sense, and partly by the rise or fall of 

 crvstals. 



The physico-chemical speculations, which played so important a 

 part in the science of rocks during the middle of the century, were 

 neglected for a time, in consequence of the opening up of a new 

 field of observation by the introduction of the microscope ; but of 

 late years we have returned to these speculations with renewed 

 vigour, 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 

 analyses have been made, and the problem of the origin of petro- 

 graphical species has resolved itself into the question of the evolu- 

 tion of the magmas. Especially noteworthy is the stimulus given 

 to the chemical side of petrology by the magnificent work of the 

 United States Geological Survey. "We have now some four or five 

 new and original classifications of igneous rocks largely based on the 

 analyses of Clarke, Hillebrand, and their assistants, and the cry is — 

 * still they come ! ' But the authors of these analyses have hitherto 

 refrained — perhaps wisely — from attempting any general classifica- 

 tion 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 every petrographer should not have his own classification and 

 his own method of graphical representation. 



