EISE OF PETROLOGY AS A SCIENCE 253 



beginnings in this had been made by Brewster, Nicol, and 

 other physicists, and the mineralogists had commenced 

 to study minerals from this viewpoint. Especially 

 Des Cloiseaux had devoted himself to determining the 

 optical properties of many minerals, and the writer, 

 when a student in the laboratory of Rosenbusch in 1890, 

 well recalls the tribute that he paid to the work of 

 Des Cloiseaux for the aid which it had afforded him in his 

 earlier researches in petrography. 



The twenty years following the publication of the 

 texts of Rosenbusch and Zirkel may be characterized as 

 the era of microscopical petrography. A distinction is 

 drawn here between the latter word and petrology, a 

 distinction often overlooked, for petrography means lit- 

 erally the description of rocks, whereas petrology denotes 

 the science of rocks. As time passed the broader and 

 more fundamental features of rocks, especially of igneous 

 and metamorphic 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 



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