PROTEROZOIC ROCKS IN THE EAST AND WEST 341 



contemporaneous. In most of these localities the Proterozoic rocks 

 are unconformable beneath overlying formations, and above the 

 Archean where that is shown. In much of the northwest, however, 

 there is conformity between the Proterozoic and the Cambrian, 

 according to present interpretations. 



In the Canyon of the Colorado, pre-Cambrian formations are 

 well exposed. The Proterozoic (Grand Canyon) group, more than 

 10,000 feet in thickness, rests unconformably on the Archean, and 

 is in turn covered unconformably by the Cambrian. Here, as in 

 Montana, a few fossils have been found. 



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

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

 ly classed as Archean. Their position is shown in Fig. 295. These 

 metamorphic rocks include some that were sedimentary, and some 

 that were igneous. A part of them are probably Proterozoic, but 

 the Proterozoic, Archean, and metamorphic Paleozoic rocks have 

 not been fully differentiated. 



Summary. While the correspondence of the Proterozoic rocks 

 in these various regions with those of the Lake Superior region is 

 not generally very close, it may be pointed out again that close 

 correspondence is not to be expected, even if the rocks of different 

 localities were contemporaneous in origin. The phases of sedi- 

 mentation taking place about any land mass at any time depend 

 largely on the height of the land, the exposure of its coasts, climate, 

 and the character of the formation suffering erosion. These various 

 factors were as likely to be dissimilar as similar about the various 

 centers of sedimentation. Igneous rocks form a not inconsiderable 

 part of the Proterozoic systems, and there is no apparent reason 

 why igneous activities in different regions should correspond either 

 in time or in the nature of their products. Even deformations of 

 the crust, which are the basis for the separation of the rocks into 

 systems, need not have been the same in different regions. It fol- 

 lows (i) that the number of Proterozoic systems bounded by uncon- 

 formities may not be the same in all regions; (2) that the thick- 

 nesses of the various systems may vary greatly; (3) that there 

 need have been no close correspondence in the sorts of rock in 

 different regions at the outset; and (4) that they may have been 

 metamorphosed unequally since their deposition. Dissimilarity of 

 the Proterozoic in different regions was, therefore, to have been 

 anticipated. 



