Ixxiv PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. [May T9OI, 



repulsion between the f^romagnesian and alumino-alkaline con- 

 stituents. Dark rocks rich in the former, and light rocks rich 

 in the latter, represent the extreme forms of many intermediate 

 types ; and Prof. Brogger has recently proposed that this should 

 receive expression by the application of the terms melanocratie 

 and leucocratic to these two strongly-contrasted varieties. 



The results already obtained leave no doubt that a properly- 

 directed series of experiments will throw great light on the laws 

 which control the formation of minerals during the consolidation 

 of igneous rocks. The classic researches of Prof. Pouque and 

 M. Michel Levy on the synthesis of such rocks as basalt, andesite, 

 and nephelinite by pure igneous fusion show that we can control 

 the necessary physical conditions, and that the whole subject, so 

 far at least as these rocks are concerned, lies within the range o^f 

 experiment. 



The work of Morozewicz, to which T have directed attention in 

 another place, may be mentioned as proving that a rich harvest of 

 results may be confidently anticipated from experimental work in 

 this direction. 



To return to the question of the order of consolidation of minerals 

 in igneous rocks. If the solution-theory be true, no order baset! 

 solely on a consideration of the properties of the minerals can hold 

 good in all cases. In the case of aqueous solutions of two sub- 

 stances the order of separation, as pointed out by Bunsen, depends 

 on the relative proportions of these two substances. This subject, 

 so far as alloys, fused salts, and aqueous solutions are concerned, 

 ■was investigated with great skill by Prof. Guthrie, the import- 

 ance of whose work on alloys has been brought into prominence 

 of late by the researches of Roberts- Austen, Le Chatelier, Osmund, 

 J. E. Stead, Heycock & Neville, Alder Wright, and others. 



It is too early yet to discuss the full bearing of this recent work 

 on petrographical questions, but it is impossible to examine the 

 beautiful photographs which illustrate the structure of alloys, such, 

 for example, as those accompanying the Pifth Eeport of the Alloys 

 Research Committee,^ or those illustrating Stead's paper on iron 

 and phosphorus,^ or Heycock & Neville's paper on gold-aluminium 

 alloys,^ without being struck by the resemblf>nce of many of these 

 structures to those met with in rocks. 



^ Fifth Eeport by Sir William Roberts-Austen, Proc. Inst. Meeh. Eng. 

 1899, p. 35. ^ Journ. Iron & Steel Inst. to]. Iviii (1900) p. 60. 



3 Phil. Trans. Roy. Soc. vol. exciv (1900) A, pp. 201-32. 



