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SCIENCE 



[N. S. Vol. XXXIV. No. 873 



tributions to this problem will always rank 

 among the classics of geology. The great 

 majority of his followers, however, have 

 been content to record and compare the re- 

 sults of observation without pushing their 

 inquiries farther; and indeed the name 

 "petrography," often applied to this line 

 of research, correctly denotes its purely 

 descriptive nature. A very large body of 

 facts has now been brought together, and 

 may be found, collated and systematized 

 by a master-hand, in the monumental 

 work of Eosenbusch. Beyond their in- 

 trinsic interest, the results thiis placed on 

 record must be of the highest value as fur- 

 nishing one of the bases upon which may 

 eventually be erected a coherent science of 

 igneous rocks and igneous activity. 



In earnest of this promise, recent years 

 have witnessed a very marked revival of 

 interest in what we must call at present 

 the more speculative aspects of petrology. 

 This manifests itself on the side of the 

 petrographer in a growing disposition to 

 seek a rational interpretation of his obser- 

 vations in the light of known physical prin- 

 ciples, and on the side of the field geologist 

 in a more constant regard for the distribu- 

 tion, mutual associations, and mode of oc- 

 ciirrence of igneous rocks. I will add, 

 as another hopeful sign of the times, a de- 

 cided rapprochement between the labora- 

 tory and the field, too often treated in 

 practise as distinct departments. 



As regards the former, the movement 

 which I have noticed is merely a return 

 to the standpoint of Sorby, the father of 

 modern petrologj^ It is true indeed that, 

 before his time, the problem of the origin 

 of igneous rocks had engaged the inge- 

 nuity of Scrope and Darwin, of Bunsen 

 and Durocher, and many others; and the 

 bold speculations of the heroic days of 

 geology have justly exercised a lasting 

 influence. The petrologist of to-day, 



however, has at his command a much 

 ampler range of information than was pos- 

 sessed by his predecessors. In addition 

 to the rich store of petrographical data 

 already mentioned, he can press into serv- 

 ice, on the one hand, the results of phys- 

 ical chemistry and, on the other, much 

 additional knowledge which has been 

 gathered concerning the structure of the 

 earth's crust and the distribution of 

 various rock-types, both in space and in 

 time. Either of these branches of the sub- 

 ject would furnish material for a much 

 longer address than my assurance could 

 venture or your complacence would en- 

 dure. I have chosen the geographical as- 

 pect of petrology; but, before proceeding 

 to this, I will say a few words concerning 

 the experimental side. 



DATA FROM THE EXPERIMENTAL SIDE 



That the modern developments of phys- 

 ical chemistry, starting from the phase 

 rule of Willard Gibbs, must in theory fur- 

 nish all that is necessary to elucidate the 

 crystallization of igneous rock-magmas, 

 has long been perceived by some petrol- 

 ogists. This recognition is in itself an ad- 

 vance. Natural rock-magmas, however, 

 are far more complex solutions than those 

 which chemists have employed in working 

 out their laws, and the problem in its en- 

 tirety is of a kind almost to daunt inquiry. 

 Despite the courageous attempt made by 

 Professor Vogt, whose enthusiastic lead 

 has done so much to inspire interest in the 

 subject, it seems clear that the application 

 of the laws of chemistry to the particular 

 class of cases with which the petrologist is 

 concerned demands as a prerequisite a 

 large amount of experimental work in the 

 laboratory. The high melting-points of 

 the rock-forming minerals, their extreme 

 viscosity, and other specific properties 

 render such work extremely difficult and 



