

THE PROTEROZOIC ERA 475 



found in the Proterozoic of Canada, 2 that glacial beds of somewhat 

 uncertain age late Proterozoic or early Cambrian are also 

 reported from Norway, 1 and that glacial formations of early Cam- 

 brian (or late Proterozoic) age exist in China. 2 Glacial formations 

 are singularly out of harmony with the conceptions of the climate 

 of early geologic time which have prevailed until recent times. 



Map studies. Map studies should be carried on in connection with the 

 chapters on the Archeozoic and Proterozoic. For this purpose, numerous 

 folios of the U. S. Geological Survey are especially serviceable; so also are 

 some of the maps and sections in various Reports of the same Survey, and of 

 certain State Geological Surveys. 



Some of the folios which are especially valuable for this purpose are the 

 following; Arizona, Bradshaw Mountain, Clifton; Colorado, Needle Moun- 

 tains, Ouray, Rico; District of Columbia, Washington; Maryland, Patuxent; 

 Michigan, Menominee; North Carolina, Asheville, Cranberry, Mount Mitchell, 

 Nantahala, Pisgah; Virginia, Harpers Ferry; Wyoming, Absaroka, Bald 

 Mountain-Dayton, Cloud Peak-Fort McKinney, Hartville, Sundance. These 

 folios show the structure and the relations of the Archean to the Proterozoic 

 (Algonkian) at divers points, and the relations of both to younger rocks. 

 These relations should be noted. The Structure Section Sheets of the folios 

 afford the means for determining, more or less closely, the sequence of 

 geologic events in the region concerned, such as the dates of deformations, 

 the dates of igneous intrusions or extrusions, the dates of faulting, etc. The 

 texts of the folios give brief descriptions of the formations, and concise state- 

 ments concerning the sequence of geologic events in the regions concerned. 



1 Reusch, Norges geologiske Undersoegelse : Det nordlige Norges Geologi, 

 1891. Also Strahan, Quar. Jour. Geol. Soc., Vol. 53, 1897, pp. 137-146. 



2 Willis, Year-Book No. 3, Carnegie Inst., p. 382. 



