Mat 28, 1920] 



SCIENCE 



531 



taught for a year in tlie Brooklyn Polytechnic 

 Institute, and then became interested in geol- 

 ogy and petrography as an assistant field- 

 worker for the U. S. Geological Survey in 

 the Yellowstone National Park; carrying on 

 studies in mineralogy and petrography in 

 Professor Penfield's laboratory, and afterwards 

 with Eosenbusch in Heidelberg and with 

 Lacrois in Paris. For nine years he was an 

 assistant and special expert on the U. S. Geo- 

 logical Survey, and since 1904 a geologist in 

 this service. 



Although a successful teacher of physical 

 geology to undergraduate students, his special 

 interests were in petrology, which he taught 

 to graduate students, and to which he devoted 

 more than half of his time and most of his 

 thought, as may be seen in his publications. 

 His research work was almost wholly petro- 

 logical. Beginning with his observations of 

 igneous rocks in the Yellowstone Park, he 

 studied independently, and in conjunction 

 with W. H. Weed, the districts of Castle 

 Mountain, Judith river basin, the Highwood 

 and Little Belt Mountains, and other local- 

 ities in Montana; and he contributed numer- 

 ous papers on the petrography of New Hamp- 

 shire in the region of Squam Lake. He was 

 joint author with Cross, Iddings and Wash- 

 ington of a Quantitative System of Classifica- 

 tion of Igneous Rocks. 



Professor Pirsson was especially successful 

 in the preparation of test-books. His ele- 

 mentary work on " Rocks and Rock Minerals," 

 wi'itten for a course of instruction without 

 the use of microscopical methods of diagnosis, 

 has been in general use for the past twelve 

 years. Later he prepared a text-book for his 

 undergraduate class in physical geology which 

 is highly esteemed and widely used; the his- 

 torical part of the volume having been written 

 by Professor Schuchert. A more elementary 

 form of the work was under consideration 

 shortly before his death. He had begun an 

 elementary petrography which was left im- 

 finished, greatly to the regret of teachers of 

 the subject. 



In addition to being a careful observer and 

 a painstaking and industrious student, he was 



methodical and systematic in his work, and 

 thorough in his treatment of a subject. 

 Moreover, he recognized the importance of 

 emphasizing fundamental principles. He was 

 an associate editor of the American Journal 

 of Science from 1897 to the time of his death. 

 There was a definiteness in his conceptions 

 and in his statements that rendered his teach- 

 ing effective and commanded the respect of 

 his students. Confident in his own judgment, 

 and tenacious of his convictions, he was at 

 the same time considerate of the opinions of 

 others and conscientious in his dealings with 

 them. 



By temperament cautious, he was reserved 

 in his intercourse with strangers, but genial 

 and outspoken in the company of friends. A 

 man of great patience and of simple tastes, 

 he enjoyed a quiet life in the study of nature, 

 being especially fond of watching birds and 

 wild animals, and of the sport of fishing; 

 and having a photographic memory for de- 

 tails and a fine sense of humor, he was an 

 entertaining story-teller, and occasionally con- 

 tributed his experiences to Forest and Stream. 



In recognition of his scientific attainments 

 he was made a member of the National 

 Academy of Sciences, American Philosophical 

 Society, Geological Society of America, of 

 which he was vice-president in 1915; fellow 

 of the American Academy of Arts and Sci- 

 ences, Connecticut Academy of Arts and 

 Sciences, Washington Academy of Sciences, 

 Geological Society of Washington, and an 

 honorary member of the Geological Society 

 of Stockholm. 



Louis Pirsson was bom in New York City, 

 November 3, 1860, was prepared for college 

 at a private school, graduated from the Shef- 

 field Scientific School of Yale with the degree 

 of Ph.B., in 1882, and was given the degree 

 of A.M. by Yale University in 1902. He 

 studied petrography in Heidelberg and Paris 

 in 1892. In 1902 he married Eliza Trumbull 

 Brush, of New Haven, daughter of Professor 

 George J. Brush. His death, after prolonged 

 illness from rheumatism, is a severe blow to 

 the science of x)etrology, of which he was the 

 foremost teacher in this country, and a sad 



