EISE OF PETROLOGY AS A SCIENCE 257 



country, in reviewing the literature, for which the suc- 

 cessive issues of the " Bibliography of North American 

 Geology" published by the U. S. Geological Survey has 

 been of the greatest value ; the writer has been struck by 

 the fact that in the first volume containing the index of 

 papers down to and including 1891, the articles on sub- 

 jects of this nature are listed under the heading of 

 petrography, whereas in the second volume (1892-1900) 

 they are grouped under petrology and the former head- 

 ing is omitted. A justification for this is found in 

 examining the list of publications and noting their char- 

 acter. With some reason, therefore, the beginning of 

 this period may be placed as in the early years of this 

 decade. Furthermore, it was at this time that the great 

 work of Zirkel 18 began to appear, which sums up so com- 

 pletely the results of the petrographic era. Rosenbusch 19 

 was formulating more definitely his views on the division 

 of rocks into magmatic groups, as displayed by their 

 associations in the field, and using this in classification; 

 an idea which, appearing first in the second edition of his 

 "Physiographie der massigen Gesteine," finds fuller 

 development in the third and last editions of this work. 

 In this country Iddings 20 published an important paper, 

 in which the family relationships of igneous rocks and 

 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 



