298 THE EVOLUTION OF PETROLOOICAL IDEAS. 



the application of the terms me hi noc ratio atul leucocratie to these two 

 strong!}^ contrasted varieties. 



The results already obtained leave no doiil)tthata 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 Professor Foucpie and M. Michel Levy on 

 the synthesis of such rocks as basalt, andesite, and nephelinite ])y pure 

 igneous fusion show that we can control the necessary physical con- 

 ditions, and that the whole suljject, so far at least as these rocks are 

 concerned, lies within the range of experiment. 



The work of Morozewicz, to which I 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 I'ocks. If the solution theory l)e true, no order based solel}' 

 on a consideration of the properties of the minerals can hold good in 

 all cases. In the ease of aqueous solutions of two substances the order 

 of separation, as pointed out l)v P)unsen, depends on the relative pro- 

 portions of these two substances. Tliis subject, so far as alloys, fused 

 salts, and aqueous solutions are concerned, was investigated with gr<^at 

 skill ])y Professor (luthrie, the importance of whose work on aUoys 

 has been brought into prominejice of late 1)V the researches of Rol)erts- 

 Austen, Le ('hatclier, Osmund, J. E. Stead, Heycock and 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 allo3\s — such, for example, 

 as those accompanying the fifth report of the alloys research conmiit- 

 tee,^' or those illustrating Stead's paper on iron and phosphorus,'' or 

 Heycock and Neville's paper on gold-aluminium alloys,'' without being 

 struck by the resemblance of many of these structures to those met 

 with in rocks. 



Some years ago 1 directed attention to the possible application of 

 Guthrie's work on cryohydrates and eutectics to petrographical ques- 

 tions, and the experience since gained has tended rather to confirm me 

 in the views which I then expressed. Fused salts which do not act 

 chemically upon each other show, when mixed in eutectic proportions, 

 a marked tendency to form spherulitic, and what may be called micro- 

 pegmatitic, intergrowths. It has since been proved that the same is 

 true of alloys. Thanks to the kindness of Mr. J. E. Stead, I am able 

 to give two figures, drawn from photographs, which illustrate this 



« Fifth Report by Sir William Roberts-Austen, Proc. Inst. Mech. Eng. 1899, p. 35. 

 ''.Toiirn. Iron and Steel Inst., vol. Iviii (1900) j). 60. 

 <-riiil. Trans. Roy. Soc, vol. cxciv (1900) A, pp. 201-232. 



