174 



GEOLOGY. 



ments for the Proterozoic. A sequence of events which would account 

 for the existing relations is illustrated by Figs. 57 and 58. 



It is to be borne in mind that the map (Fig. 38) shows only the 

 exposed areas (so far as known) of the Archean and Proterozoic. The 



Fig. 56. — Figure showing troughs of Proterozoic rock (Al) included in Archean C£T; 

 Marquette region, Mich. Length of section, 2\ miles. (Van Hise, Bayley, and 

 Smyth, Mono. XXVIII, U. S. Geol. Surv.) 



real extent of both systems is much greater. The Proterozoic beds of 

 Wisconsin are probably continuous beneath younger formations with 



Figs. 57 and 58. — Fig. 57 shows Archean rock (At), on a portion of which Proter- 

 ozoic sediments (Al) have been deposited. Fig. 58 shows the same area after 

 deformation. The Archean rock (At) not covered by Proterozoic sediments has 

 been depressed and other sediments, a, have been deposited upon it. The result is 

 that the Archean rock (At) is nowhere exposed at the surface, and Proterozoic 

 rock (Al) appears at the surface not in association with outcrops of Archean. 



the Proterozoic beds of southwestern Minnesota, and these in turn 

 with those of Missouri and Texas (where the pre-Cambrian rock is 

 probably Proterozoic in part at least) on the south, and with those 

 of the Black Hills and Rocky Mountains (where the undifferentiated 

 pre-Cambrian of the map, Fig. 38, is at least partly Proterozoic) on 

 the west. The total area where Proterozoic rocks exist is therefore 



