GENERAL CONSIDERATIONS 685 



For western New England the general situation seems to be somewhat 

 similar to that of the Piedmont district. In both of these regions the 

 pre-Cambrian rocks Avere much folded during Paleozoic time, bu.t there 

 seems to be little^ if any, positive proof tliat they were very notably 

 folded in pre-Cambrian time. 



In Wisconsin and Minnesota deformation of older pre-Cambrian rocks 

 may generally be distinguished from that of the younger pre-Cambrian, 

 and almost undisturbed Cambrian strata rest on the pre-Cambrian. 



In the Eocky Mountain and Colorado Plateau districts of the western 

 United States two sets of pre-Cambrian rocks, representing either one 

 or two times of pre-Cambrian deformation, have been recognized in rela- 

 tively few places, as, for example, in the Grand Canyon of Arizona; 

 Needle Mountains, Engineer Mountain quadrangle, and Eocky Moun- 

 tain Park, Colorado, and Three Forks and Little Belt Mountains, Mon- 

 tana. In a few districts, as, for example, Pueblo, Colorado, and Fort 

 Benton, Montana, x\rchean pre-Cambrian rocks only are described. Al- 

 gonkian rocks only are described as occurring in a number of districts, 

 as, for example, Telluride, Eico, and Ouray, Colorado; Hartville, Wyo- 

 ming, and Philipsburg, Montana. In most of the described regions, how- 

 ever, the j^re-Cambrian is not differentiated, as, for examjole, Georgetown, 

 Silver Cliff, Pikes Peak, and Park Eange, Colorado; Medicine Bow, En- 

 campment, Laramie-Sherman, and Bald Mountain-Dayton, Wyoming; 

 Coeur d'Alene, Idaho; Bradshaw Mountains, Clifton, Globe, and Bisbee, 

 Arizona, and Van H[orn and Llano-Burnet, Texas. 



In most of the districts of the Eocky Mountain-Colorado Plateau re- 

 gion just mentioned the pre-Cambrian rocks, in part at least, are defi- 

 nitely known to have been more or less deformed in pre-Cambrian time. 

 In some of the districts the Algonkian strata are either not very notably 

 folded, as, for example. Grand Canyon, Arizona, and near Marysville, 

 Montana, or their folding was largely or wholly developed by post-Cam- 

 brian disturbances, as, for example, Philipsburg and Three Forks, Mon- 

 tana, and Ouray, Colorado. 



Enough facts have now been presented to show that, in any attempt 

 to work out the folded structures of pre-Cambrian age in North America, 

 it must be recognized that by no means all pre-Cambrian rocks, and not 

 even all the earlier ones, have been severely folded, and that a careful 

 discrimination must be made between pre-Cambrian rocks folded in pre- 

 Cambrian time and those folded later. Unless such a distinction can be 

 made, the existing strikes of folds of pre-Cambrian rocks afford no trust- 

 worthy criteria in regard to pre-Cambrian diastrophism. 



