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SCIENCE 



[N. S. Vol. XXIX. No. 736 



physical research is steadily enlarging the 

 field of petrological investigation, and the 

 advancement in the knowledge of physico- 

 chemical laws is furnishing the investigator 

 with new tools for the work and more effi- 

 cient means for attacking the problems of 

 igneous rocks. Foremost in the ranks of 

 those who have attempted the application 

 of modern conceptions of physical chem- 

 istry to the elucidation of the phenomena 

 of texture and mineral composition and of 

 the genetic relationships of igneous rocks 

 is Vogt, whose earlier studies of furnace 

 slags opened the way for the explanation 

 of many analogous phenomena in the more 

 refractory, volcanic lavas. Chief among 

 these are the apparent order of crystalliza- 

 tion of different minerals in slags, as indi- 

 cated by their shapes and relations to one 

 another as inclusions, and the relation be- 

 tween these orders and the composition of 

 the mixture from which they crystallized. 

 Vogt's observations were found to be in 

 accord with modern theories of solutions, 

 as Bunsen foretold in 1861, when he af- 

 firmed his belief that rock magmas are solu- 

 tions of silicate compounds liquid at high 

 temperatures. Vogt has called the atten- 

 tion of petrologists to these modern theories 

 as developed by Arrhenius, van't Hoff, 

 Ostwald, Gibbs, Meyerhoffer, Roozeboom 

 and others. He has also made definite 

 application of them to some of the phe- 

 nomena and relationships mentioned. His 

 publications have extended widely the hori- 

 zon of modern petrology, which by the 

 assumption of these broader, deeper phases 

 of the study of igneous rocks, and of sim- 

 ilar problems affecting metamorphic rocks, 

 has passed beyond the narrower boundary 

 of petrography, strictly so called. 



The evolution of chemistry from a state 

 of pure empiricism to one of comparatively 

 logical sequence has placed before us a col- 

 lection of coordinated laws, which, while 

 incomplete, or subject to numerous excep- 



tions, furnishes us with means to postulate 

 reactions between the constituent elements 

 of rock magmas with reasonable assurance 

 of correctness, or to explain the formation 

 of mineral compounds hitherto in a meas- 

 ure enigmatical. Much remains to be 

 more firmly established, both as to the 

 chemical character of the elements and 

 their compounds, and with regard to 

 theories relating to their reactions, and 

 even to the very nature of their existence 

 in some instances. The silicate compounds 

 constituting igneous rocks remain largely 

 uninvestigated, so far as concerns their 

 synthesis and reactions in mutual solution. 

 And the physical study of solutions and of 

 their transitions to the solidified compo- 

 nents — especially the more complex mix- 

 tures—is far from completed. The present 

 is a period of transition in the development 

 of petrology, as were also times past. But 

 the changes taking place at this time ap- 

 pear to be so many and so fundamental 

 that it may well be asked whether the older 

 methods of approach to the study of igne- 

 ous rocks should not be replaced by others 

 more in accord with present conditions of 

 knowledge of chemistry, physics and of the 

 rocks themselves. The older method, in the 

 nature of things, was, and is largely at the 

 present day, objective, and the expressions 

 of relationships or laws empirical. 



It would seem more reasonable to begin 

 a systematic study of igneous rocks with a 

 consideration: of the most fundamental 

 characteristics of the magmas from which 

 they have solidified; of their constituents, 

 together with their probable chemical reac- 

 tions and the resulting mineral compounds ; 

 of the manner in which these may separate 

 from a silicate solution, or rock magma ; of 

 the shapes they are likely to assume upon 

 crystallization and the consequent texture 

 of the rock. Processes of molecular separa- 

 tion of magmas lead to the discussion of 

 the differentiation of magma into ehem- 



