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



are enclosed, became better known; and the relations, 

 which these bear to the associations of rocks and their 

 modes of occurrence, began to be perceived, the science 

 assumed a broader aspect. The perception that rocks 

 were no longer to be regarded merely as interesting 

 assemblages of minerals, but as entities whose charac- 

 ters and associations had a meaning, increased. More 

 and better rock analyses stimulated interest on the 

 chemical side and this and the genesis of their minerals 

 led to a consideration of the magmas and their func- 

 tions in rock-making. The fact that the different kinds 

 of rocks were not scattered indiscriminately, but that 

 different regions exhibited certain groupings with com- 

 mon characters, was noticed. These features led to 

 attempts to classify igneous rocks on different lines from 

 those hitherto employed, and to account for their origin 

 on broad principles. In other words, the descriptive 

 science of petrography merged into the broader one of 

 petrology. No exact time can be set which marks this 

 passage, since the evolution was gradual. Yet for this 

 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 

 ' ' Physiographic 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 



