Louis Valentine Pirsson. 181 



petrographers of various countries were endeavoring to 

 select from the mass of detailed knowledge accumulated 

 in a few decades the criteria upon which might be based 

 a much needed contribution to the systematic classifica- 

 tion of igneous rocks. Pirsson 's accurate and extensive 

 knowledge of the rocks, his originality and good judg- 

 ment naturally made him one of a group of American 

 petrographers who jointly undertook in 1898 the task of 

 formulating an entirely new system, on a new basis, and 

 supplying the. necessary terminology. 



The group of men who accomplished this task — consist- 

 ing of Pirsson, Iddings, Washington, and the writer — 

 embraced men of different temperaments and mental 

 characteristics, with views originally conflicting on many 

 points. Success in reaching an agreement was rendered 

 possible only through the influence of qualities such as 

 Pirsson possessed in most notable degree. These quali- 

 ties were recognized by his colleagues in making him the 

 "Moderator" of numerous conferences where he was 

 charged to bring about the necessary moderation of ex- 

 treme views occasionally expressed as well as to guide 

 the discussions. His judicial temperament and construc- 

 tive ability made him a very effective member of the 

 group. 



The ideal College or University teacher is unquestion- 

 ably one who possesses enthusiasm as an instructor, an 

 extensive accurate knowledge of his subject, the power 

 of clear presentation and explanation, and, in addition, 

 that close touch with the problems of the science which 

 can only come with experience as an investigator. In 

 Pirsson this ideal was realized in high degree. His ex- 

 perience with undergraduates in chemistry, mineralogy 

 and geology extended over thirty-seven years, and with 

 graduates to about twenty-five years, largely in petrology. 



Pirsson was greatly interested in both classes of 

 students though it is probably true that the training of 

 the graduate men afforded him the greater pleasure. He 

 gave a great deal of earnest conscientious attention to the 

 effective presentation of a difficult subject to the under- 

 class men. He had his own ideas as to how this should 

 be done and his scheme is preserved in the text book soon 

 to be referred to. 



It was Pirsson's aim in presenting geology to beginners 

 to arouse their interest, not so much by a general state- 



