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



methods of classification proposed by French and Ger- 

 man geologists nearly exhaust the list. 



Introduction of the Microscope. 



The beginnings of a particular branch of science are 

 generally obscure and rooted so imperceptibly in the 

 foundations on which it rests that it is difficult to point 

 to any particular place in its development and say that 

 this is the start. There are exceptions of course, like the 

 remarkable work of Willard Gibbs in physical chemistry, 

 and it may chance that the happy inspiration of a single 

 worker may give such direction to methods of investiga- 

 tion as to open the gates into a whole new realm of 

 research, and to thus create a separate scientific field, as 

 happened in Radiochemistry. 



This is what occurred in petrology when Sorby in 

 England, in 1858, 3 pointed out the value of the micro- 

 scope as an instrument of research in geologic investiga- 

 tions, and demonstrated that its employment in the study 

 of thin sections of rocks would yield information of the 

 highest value. Others beside Sorby had made use of the 

 microscope, as pointed out by Zirkel, 4 but, as he indi- 

 cates, no one before him had recognized its value. Dur- 

 ing the next ten years or so, however, its recognition was 

 very slow and the papers published by Sorby himself 

 were mainly concerned in settling very special matters. 



As Williams 5 has suggested, the greatest service of 

 Sorby was, perhaps, his instructing Zirkel in his ideas 

 and methods, for the latter threw himself whole-heart- 

 edly into the study of rocks by the aid of the microscope 

 and his discoveries stimulated other workers in this field 

 in Germany, his native country, until the dawning science 

 of petrology began to assume form. A further step for- 

 ward was taken in 1873 in the appearance of the text- 

 books of Zirkel 6 and Rosenbusch 7 which collated the 

 knowledge which had been gained and furnished the 

 investigator more precise methods of work. It is diffi- 

 cult for the student of to-day to realize how much had 

 been learned in the interval and, for that matter, how 

 much has been gained since 1873, without an inspection 

 of these now obsolete texts. In 1863, Zirkel, who was 

 then at the beginning of his work, said in his first paper 

 presented to the Vienna Academy of Sciences 8 that if he 



