VIII. SOME NOTES ON THE GEOLOGY OF 

 SOUTHWESTERN MONTANA. 



Bv E.\RL Dou(;r,ASS. 



While exploring the Tertiary deposits of southwestern Montana, I 

 have become interested in some of the older formations, and have 

 made collections from those which contain fossils. Several of these 

 collections have been submitted to specialists, who have sent me lists 

 of the fossils, and have given their opinions concerning the ages of 

 the beds containing them. I am especially indebted to F. H. Knowl- 

 ton, Geo. H. Girty, and T. W. Stanton, of the United States Geological 

 Survey, and to Percy E. Raymond of the Carnegie Museum, for deter- 

 mining fossils and furnishing the lists, which have been of so much 

 interest to me in my geological work. 



ARCH.EAN. 



Under this head I include only the gneisses and schists which are 

 Pre-Cambrian and generally recognized as Archaean. Concerning the 

 more massive granites, regarding the age of which there is doubt, and 

 which are probably in part of later age, I have little to say. 



I have observed the Archaean rocks in the following localities : 



1. In the region between the lower and upper INIadison Valleys, 

 where the Madison River has made a long cafion through these rocks ; 



2. In the western portion of the Madison Range, and in the eastern 

 portion of the Jefferson Range, which includes the Tobacco Root 

 and the South Boulder Mountains, where they have been mapped by 

 the United States Geological Survey ;' 



3. On the western slopes of the Jefferson Range ; 



4. Along the lower canon of the Ruby (Passamari) River nearly 

 west of Old Baldy Mountain ; 



5. In the Ruby Mountains; 



6. In the vicinity of Rochester. This region is between Table 

 Mountain on the north and McCarty's Mountain on the south, and 



' Atlas Folio No. 24. 



407 



