180 Louis Valentine Pirsson. 



the current designation of certain areas as "Petrographic 

 provinces." That of central Montana as established by 

 Pirsson is one of unusual general interest. 



Pirsson 's increasing University duties prevented kis 

 entering further on field work of considerable magnitude. 

 But he turned to the study of promising areas in New 

 Hampshire near Squam Lake, where he spent many 

 summer vacations. In association with H. S. Washing- 

 ton he undertook the study of the Belknap Mountains, 

 south of Lake Winnepesauke and of Red Hill on the 

 eastern side of Squam Lake. A similar study of Tri- 

 pyramid Mountain, on the south side of the White Moun- 

 tains, was undertaken in collaboration with William 

 North Rice. 



In addition to the districts with which his published 

 articles dealt he gained a first-hand knowledge of, and 

 examined with considerable care, many other important 

 localities and entertained for a time a project of extended 

 research in New Hampshire. But the increasing de- 

 mands of college duties interfered with this plan. 



In these researches modern methods of petrographic 

 study brought out the existence of many important rock 

 types with interesting genetic relationships which earlier 

 work had not disclosed. In all this -work Pirsson was 

 always mainly interested in making a contribution to our 

 knowledge of the origin of the chemical and mineral dif- 

 ferences in igneous rocks — petrogenesis. He had little 

 interest in petrographic description per se. 



Pirsson 's field researches were mainly confined to 

 Montana and New Hampshire but he was quick to see the 

 importance of chance observations in many directions 

 and interested others to follow up the more promising 

 leads where he could not do so. Thus, through a visit to 

 Bermuda he learned of the geological results of a deep 

 boring for water, and by cooperation with Drs. Vaughan 

 and Cushing of the U. S. Geological Survey and Dr. 

 Thomas of the British Survey an important contribution 

 was made to our knowledge of this mid-Atlantic island. 



The philosophical side of the investigator was always 

 prominent in Pirsson 's publications. The Montana work 

 led to special discussions of "Complementary rocks 

 and radial dikes," a review of significant relations of the 

 "Phenocrysts of igneous rocks," and the paper above 

 mentioned on the "Petrographic Province of Montana." 



His main work was done in a period when thoughtful 



