258 A CENTURY OF SCIENCE 



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, Gushing, Daly, Lane, Lawson, 

 Lindgren, Pirsson, J. P. 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 the Journal during the last twenty years show 

 this evolution and growth of petrological 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 w r ith those of a purely 

 descriptive character. 24 Thus we find discussions by 

 Becker on the physical constants of rocks, on fractional 

 crystallization, and on differentiation; by Cross on 

 classification; by Adams on the physical properties of 

 rocks ; by Daly on the methods of igneous intrusion ; by 

 Wright on schistosity; by Fenner on the crystallization 

 of basaltic magma; by Bowen on differentiation by 

 crystallization; by the writer on complementary rocks 

 and on the origin of phenocrysts ; by Smyth on the origin 

 of alkalic rocks ; by Murgoci on the genesis of riebeckite 

 rocks ; and by Barrell on contact-metamorphism. These 

 may serve as examples, selected almost at random, from 

 the files of the Journal, and we find with them articles 

 descriptive of the petrology of many particular regions, 

 which often contain also matter of general interest and 

 importance, such as papers by Lindgren on the grano- 

 diorite and related rocks of the Sierra Nevada; by 

 Ransome on latite ; by Cross on the Leucite Hills ; by 



