L. V. Pirsson — Rise of Petrology as a Science. 231 



the derivation of diverse groups from a common magma 

 by differentiation are clearly brought out. The funda- 

 mental problems underlying the genesis of igneous rocks 

 had now been clearly recognized, and with this recogni- 

 tion the science passed into the petrologic phase. 

 Brogger 21 also had ascribed to the alkalic rocks of South 

 Norway a common parentage and had pointed out their 

 regional peculiarities. 



From this time forward an attempt may be noted to find 

 an analogy between rocks and the forms of organic life 

 and to apply those principles of evolution and descent, 

 which have proved so fruitful in the advancement of the 

 biological sciences, to the genesis and classification of 

 igneous rocks. This, perhaps, has on the whole been 

 more apparent than real, in the constant borrowing of 

 terms from those sciences to express certain features and 

 relationships observed, or imagined, to obtain among* 

 rocks. Nevertheless, the perception of certain relations 

 which we owe so largely to Rosenbusch and to Brogger 22 

 has proved of undoubted value in furnishing a stimulus 

 for the investigation of new regions, and in affording 

 indications of what the petrologist should anticipate in 

 his work. 



Thus, the labors of the men previously mentioned, with 

 those of Bayley, Bascom, Cushing, Daly, Lane, Lawson, 

 Lindgren, Pirsson, J. F. Williams, Washington, and 

 others, have thrown a flood of light upon the igneous 

 rocks of this continent, and has made it possible to draw 

 many broad generalizations concerning their origin and 

 distribution. Thus, the differentiated laccoliths of Mon- 

 tana 23 have been of service in affording clear examples of 

 the process of local differentiation. Many papers pub- 

 lished in this Journal during the last twenty years show 

 this evolution and growth of penological ideas. The 

 contributions from American sources during this later 

 period, and of which those in the Journal form a consid- 

 erable fraction, have indeed been of great weight in 

 shaping the development and future of the science. 



By referring to the files of the Journal, it will be seen 

 that they cover a continually widening range of subjects 

 concerning rocks, and articles of theoretical interest are 

 more and more in evidence, along with those of a purely 

 descriptive character. 24 Thus we find discussions by 

 Becker on the physical constants of rocks, on fractional 



