OBSERVED STRIKES OF PRE-CAMBRIAN FOLDING AND FOLIATION 697 



"The granites are flexed and contorted in every possible direction and con- 

 tain simple bands of micaceous and chloritic schists which denote the original 

 planes of stratification." 



In the Gallatin Eange of Yellowstone Park (Hague, 1899) a relatively 

 small area of pre- Cambrian schist shows a general north-south strike. 



MONTANA AND IDAHO 



There are several areas of j)re-Cambrian rocks in the western half of 

 Montana for which more or less structural information is available. 



In the Little Belt Mountains (Weed, 1899) an area of Archean has 

 steep dips and a general east-west strike. Eesting on the Archean are 

 very moderately folded Algonkian and Paleozoic strata. 



A small area of Archean gneiss and schist in the Fort Benton Quad- 

 rangle (Weed, 1899) shows steep dips and very variable strikes. 



The geological map of the Three Forks Qrradrangle (Peale, 1896) 

 shows large areas of Archean gneisses, but their structure is not de- 

 scribed. There are also Algonkian (?) marble and schists which were 

 metamorphosed and folded (structural data not given) before the dep- 

 osition of an unaltered Algonkian sedimentary series, which latter is 

 intricately infolded with Cambrian and Cretaceous strata. 



In the Mar3'Sville district (Barrell, 1907) near Helena, Algonkian 

 strata are only gently folded with very irregular strikes. The prevailing- 

 strike seems to be northwest, but Barrell speaks of a cleavage structure 

 with a northeast trend. 



In the Philipsburg Quadrangle (Calkins and Emmons, 1913) Algon- 

 kian strata are extensively exposed with exceedingly variable strikes and 

 with only a general parallelism with the late Cretaceous folds, so that in 

 part the folding may be of pre-Cambrian age. 



The great body of Algonkian strata in Glacier Park has a long, wide, 

 gentle synclinal structure, with nearly north-south strike, but this struc- 

 ture was probably developed at the time of the thrust-faulting in the 

 early Tertiary. 



The Coeur d'Alene district of Idaho (Ransome and Calkins, 1908) 

 contains much pre-Cambrian stratified rock notably folded with very 

 irregular strikes, but with a northwest trend somewhat prevalent. 



CONCLUSIONS^ REGARDING THE CORDILLERAN REGION 



From the data above presented it is evident that there is no predom- 

 inant pre-Cambrian structural trend through the Cordilleran region of 

 the United States approximately parallel (north-northwest) with the 

 trend of the Rocky Mountains. A glance at the accompanying map will" 



XLVI — Bull. Geol. Soc. Am., Vol. 34, 1922 



