L. V. Pirsson — Rise of Petrology as a Science. 227 



the science of rocks. As time passed the broader and 

 more fundamental features of rocks, especially of igneous 

 and met amorphic rocks, in addition to their mineral 

 constitution, were more studied and gained greater recog- 

 nition, petrography gradually became a department of 

 the larger field of petrology — the science of to-day. 



The use of the microscope, as soon as the method 

 became more generally understood, opened up so vast a 

 field for investigation that at first the study and descrip- 

 tion of the rocks seemed of prime importance. This was 

 natural, for hitherto the finer grained rocks had for the 

 most part defied any adequate elucidation and here was a 

 key which enabled one to read the cipher. A flood of lit- 

 erature upon the composition, structure, and other char- 

 acters of rocks from all parts of the world began to 

 appear in ever increasing volume. The demands of the 

 petrographers for a greater and more accurate knowledge 

 of the physical and optical constants of minerals stimu- 

 lated this side of mineralogy, and increasing attention 

 was given to investigations in this direction. No definite 

 line between the two closely related sciences could be 

 drawn, and a large part of the work published under the 

 heading of petrography could perhaps be as well, or 

 better, described under the title of micro-mineralogy. 

 To some, in truth, the rocks presented themselves simply 

 as aggregates of minerals, occurring in fine grains. 



The work of the German petrographers attracted 

 attention and drew students from all parts of the world 

 to their laboratories, especially to those of Zirkel and 

 Eosenbusch. The great opportunities, facilities, and 

 freedom for work which the German universities had 

 long offered to foreign students of science naturally 

 encouraged this. In France a brilliant school of penolo- 

 gists, under the able leadership of Michel-Levy and 

 Fouque, had arisen whose work has been continued by 

 Barrois, Lacroix and others, but the rigid structure of 

 the French universities at that period did not permit 

 of the- offering of great inducements for the attendance 

 of foreign students. The work of the French petrog- 

 raphers will be noticed in another connection. 



In Great Britain, the home of Sorby, the new science 

 progressed at first slowly, until it was taken up by All- 

 port, Bonney, Judd, Rutley, and others. In 1885 the 

 evidence of the advance that had been made and of the 



