THE PROTEROZOIC ERA. 213 



igneous. In some places they are so far metamorphosed as to have 

 lost all trace of their original texture, while in others the change is 

 much less complete. Within this belt of crystalline rocks, Proterozoic 

 formations are believed to occur, but they have not been generally 

 differentiated from the Archean on the one hand, and from the meta- 

 morphic Paleozoic rocks on the other. 



Fig. 88. — Figure showing the Archean, Proterozoic, and later formations at one point 

 in the Grand Canyon of the Colorado in northern Arizona. The lowermost non- 

 stratified formation is Archean. The horizontal beds in the upper part are Paleozoic, 

 their basal portion being Cambrian. The tilted beds between the Archean and 

 Paleozoic are Proterozoic, (At wood.) 



In the Green mountains of Vermont, there appear to be two series (the Mendon 

 and the Mount Holly) of sedimentary or meta-sedimentary rocks of complicated 

 structure, unconformable beneath the Cambrian. 1 Patches of pre-Cambrian rock 

 are known in thirteen places in western Massachusetts, but they are separated 

 with difficulty from the Cambrian, Silurian, and Devonian beds which were meta- 

 morphosed with them. They consist of gneisses, schists, and crystalline lime- 

 stone 2 (Fig. 89). In southeastern New York there appear to be pre-Cambrian 

 metamorphic rocks of complicated structure (Fordham gneiss) closely associated 



1 Whittle, cited by Van Hise, 16th Ann. Rept. U. S. Geol. Surv., Pt. I, p. 827. 



2 Ibid., p. 830; and Emerson, Holyoke, Mass., folio, U. S. Geol. Surv. 



