IxXXii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9OI, 



test. The principle of Gouy & Chaperon is even more unsatis- 

 factory. Durocher's liquation-theory is, perhaps, more promising, 

 but until it has been proved by actual experiment that there is a 

 real analogy between baths of fused metals and silicate-magmas it 

 cannot be said to rest upon an assured basis. Faraday's researches 

 on lead-glass certainly suggest that gravity may act differen- 

 tially on the constituents of silicate-magmas, independently of the 

 principle of Gouy & Chaperon. Thus he found that glass taken 

 from the top of pots not more than 6 inches deep might have 

 a density of 3'28, while that from the bottom might have a 

 density of 3-85 ; but there is some doubt as to whether the consti- 

 tuents were ever uniformly mixed in the molten state, and if not, 

 whether sufficient time was allowed for diffusion to establish homo- 

 geneity. It is certain, however, that they were uniformly mixed in 

 the solid state, and the experiments are therefore of great interest ; 

 for, if they do not prove differentiation in a molten mass, they prove 

 that an uniform solid mass may become diflferentiated as it liquefies, 

 by a kind of liquation-process analogous to that which takes place 

 in the extraction of silver from copper. 



Prof. Iddings has carefully considered the chemical compositions 

 of groups of rocks, belonging to several different petrographical 

 provinces, from the point of view of the differentiation-hypothesis, 

 and has arrived at the conclusion that ' the simple-oxide molecules 

 shift about independently of one another to a great extent.' If this 

 conclusion be correct, it is clear that the phenomena cannot be 

 explained by the hypothesis of a differentiation solely connected 

 with the formation of known minerals ; but this view does not appear 

 to be accepted by Prof. Brogger, who believes 



' that the process of differentiation must be referred to magmatic diffusion of 

 definite chemical compounds to and from the cooling surface ; further, that 

 these diffusion-phenomena in all probabihty stand in direct relation to the 

 order of crystallization of minerals in the corresponding magma; and lastly, 

 that the order of crystallization, the nature of the differentiation, and the 

 sequence of eruptions are all closely related phenomena.' 



Differentiation dependent upon crystallization rests on a somewhat 

 firmer basis, and it was this kind of differentiation that first attracted 

 my attention. Mr. Clough, while mapping the Cheviot district, 

 proved that the widespread series of andesitic lavas is cut by a 

 number of quartz-felsite dykes. Why did quartz-felsite succeed 

 andesite in the Cheviot district ? This was the question which kept 

 continually recurring to me during my examination of the rocks of 



