REVIEW OF EARLIER INVESTIGATIONS 441 



buttes when there was still some danger from raiding parties of Indians. 

 Dr. Dawson ascended both East and West buttes, collected specimens of 

 the igneous rocks, and made valuable notes on the associated sedimentary 

 strata. He noted the upheaval of the sediments, their quaquaversal dips 

 from the intrusive centers, and described the features of the later named 

 laccolites or laccoliths, without constituting them into a new and dis- 

 tinctive type. 



The introduction of the term laccolite remained for G. K. Gilbert in 

 1877. The new name afforded an indispensable one for descriptive geol- 

 ogy. Dr. Dawson likened the Sweet Grass upheavals to von Buch's 

 "craters of elevation." The term is not a specially happy one, in that 

 we associate craters with volcanoes, and there are no effusive rocks what- 

 ever in the Sweet Grass Hills. They are purely laccoliths, sills and dikes, 

 and uptilted sediments. 



RESEARCHES OF FRANK D. ADAMS, W. H. WEED, AND L. V. PIR8S0N 



Dr. Dawson made another hasty visit to West Butte in the years 1882- 

 81, during his studies of the region of the Bow and Belly rivers. In 

 connection with his report, published in 1881,^ Frank D. Adams con- 

 tributed some microscopic determinations of the principal igneous rocks, 

 recognizing them as intermediate or transitional between trachytes and 

 andesites — a general character which we have no reason to modify today, 

 although, on account of their intrusive nature and to be in accord with 

 the usage of Weed and Pirsson, we may speak of the same types as syenite 

 porphyry and diorite porphyry. 



The more complete and detailed collections of the writers have some- 

 what extended the acidic and basic limits and Jiave l)rought to light some 

 types rich in soda, such as also appear in the intrusive centers studied 

 elsewhere by Weed and Pirsson. 



Besides the commoner run of the rocks. Dr. Dawson collected speci- 

 mens from a peculiar basic dike standing out like a wall in the plains 10 

 miles north of East Butte. It is very rich in biotite and was referred by 

 him to von Cotta's kersanton. In the later paper by Weed and Pirsson 

 tliis is more accurately described as minette. We liave found the rock 

 l)oth in sills and dikes in a number of places. With the help of George 

 M. Fowler, geologist on the staff of the Anaconda Chopper Company, we 

 have identified several localities of tinguaites and more acidic ffigirite- 

 bearins: rocks in East Butte.'^ 



« Geological Survey of Canada. Report for 1882-1884, pp. 16C-18(', 45('-48C. ISSf). 



■^ The data for this paper were obtained in the following wa.v : The two writers spent 

 a week in the Hills in May, 1918, in company with George M. Fowler, who was in 

 charge of some local explorations. The previous fall the junior writer had essentially 



