THE CAMBRIAN PERIOD 



487 



The areas where the Cambrian formations are exposed are not 

 to be confounded with the areas where they actually exist. The 

 Cambrian formations are exposed, for example, in Wisconsin, in 

 Missouri", and in Texas; but the strata of Texas are doubtless con- 

 tinuous, beneath younger formations, with those exposed in Mis- 

 souri, and those of Missouri with those of Wisconsin, and these in 

 turn with those of the Black Hills on the west, and with those of 

 New York on the east (Fig. 361). 



Position of outcrops. The map (Fig. 358) showing the areas 

 where the Cambrian system is now exposed, reveals several points 

 of significance. (1) . Many of the outcrops occur in association with 

 outcrops of the Archean and Proterozoic systems (Fig. 341). In 

 some places, the exposed Cambrian lies along the border of the ex- 

 posed parts of these older systems on one side only, while in others 



Fig. 362. Diagram showing the positions of outcrops as determined by 

 folds. The anticline at the right is represented as having been trun- 

 cated in such a way as to expose the Cambrian, -0, in a single belt ; that 

 at the left has been cut so as to expose pre-Cambrian formations, A, 

 and the Cambrian, -G, outcrops in two belts, one on either side of the 

 older formation. 



Fig. 363.- 



Figure to illustrate two sorts of isolated occurrences of Cambrian, 

 surrounded by older formations. 



it completely surrounds them. This distribution is not peculiar to 

 the Cambrian, but is characteristic of most formations as com- 

 pared with those of greater age. (2). The exposed areas of 

 Cambrian in the Appalachian Mountains * occur in parallel or sub- 

 1 See folios of the U. S. Geol. Surv. 



