Louis Valentine Pirsson. 3 79 



plunged at once into a series of broad petrologic investi- 

 gations in Montana. Those researches were a direct out- 

 growth of his experience in the Geological Survey, 

 especially in 1890, when a glimpse of the Crazy Mountains 

 on the northern border of the Livingston Quadrangle 

 indicated the rich field for study lying farther north in 

 similar laccolithic mountain groups, east of the Rocky 

 Mountains proper. The investigations in the field were 

 carried out in 1893, 1894 and 1896, in association with 

 W. H. Weed of the Geological Survey, when the Castle, 

 Little Belt, Highwood and Judith Mountain groups were 

 examined. Mr. "Weed examined the Bearpaw and Little 

 Rocky Mountains alone, in 1895, and collected specimens 

 of the igneous rocks to be studied by Pirsson. The de- 

 scription of these mountain groups, especially as to their 

 igneous geology and petrography, issued by the Geologi- 

 cal Survey in a number of joint papers, and in periodicals, 

 together with several articles by Pirsson alone, forms one 

 of the most important contributions to American petrol- 

 ogy. Pirsson's share in this work was a large one, ex- 

 tending beyond the purely petrologic discussion. 



The various publications giving the results of this work 

 are among the most widely known and highly valued of 

 all penological contributions of the past twenty-five 

 years. Their importance lies in several directions. 

 They gave a great deal of evidence concerning the nature 

 of laccolithic type of igneous intrusion as the primary 

 cause of isolated mountain groups in a plain country and 

 confirmed the general ideas of Gilbert on this subject. 

 Pirsson's powers as an observer combined with his skill 

 in graphic illustration and in clear description render the 

 petrologic portions of these reports of special value. 



The chemical analyses, mostly made by Pirsson, and 

 the microscopical study of the rocks, revealed many in- 

 teresting rock types, some of them quite new to science. 

 The character and relations of these rocks were ac- 

 curately established and clearly described, furnishing an 

 important contribution to, systematic petrography. But 

 Pirsson was keenly alive to the questions of genesis and 

 secured from this material many valuable hints as to the 

 difficult subject of magmatic differentiation and genetic 

 relations. 



Finally he presented a survey of the established facts 

 which went far to substantiate and illustrate the idea of 

 regional characteristics of igneous rocks which lead to 



