472 GEOLOGY 



long and nearly as broad, numerous dikes penetrate the clastic 

 beds, and furnish good illustrations of the metamorphosing ef- 

 fects of igneous intrusions. 



In the Adirondack region, Pre-Cambrian rocks make up a large 

 part of the Adirondack Mountain mass. They belong to two 

 groups: (1) The central mass of the mountains, consisting of igneous 

 rocks which were intruded into (2) a series of pre-Cambrian sedi- 

 mentary rocks which surround and cover the base of the igneous 

 mass. 



The Cordilleran region. The axial cores of many of the older 

 mountain ranges of the West are believed to be of Archean rock. 

 In many of them there are thick series of sedimentary or meta-sedi- 

 mentary rocks overlying the Archean and surrounding its outcrops, 

 overlain in turn by Cambrian or younger strata. Rocks referred 

 to the Proterozoic are found in the Medicine Bow and some other 

 mountains of Wyoming; in the Bridger, Little Belt, Lewiston, and 

 Livingston ranges of Montana, where some of them are fossiliferous; 

 in British Columbia; in the Wasatch and certain lesser ranges of 

 Utah; in several of the ranges of Nevada and Colorado, and in the 

 Grand Canyon of the Colorado in Arizona. In most if not all of 

 these localities, sedimentary beds predominate, but are accom- 

 panied by igneous rocks which are in part contemporaneous. The 

 thickness of the Proterozoic rocks in these various localities is often 

 great, and in most places they are unconformable on their base, 

 and beneath overlying formations. In much of the northwest, 

 however, there is conformity between the Proterozoic and the 

 Lower Cambrian, according to present interpretations. 



In the Canyon of the Colorado (Arizona) the pre-Cambrian for- 

 mations are well exposed. The Proterozoic (Grand Canyon) group, 

 more than 10,000 feet in thickness, here rests unconformably 

 on the Archean, and is in turn covered unconformably by the 

 Cambrian. The group is itself divisible into two systems by a 

 slight unconformity. Here, as in Montana, a few fossils have been 

 found. 



In the eastern part of the United States. 1 There are large areas 

 of metamorphic rock in the eastern part of the United States, 



1 See Bull. 86, U. S. Geol. Surv., and folios, U. S. Geol. Surv. 



