254 GEOLOGY. 



the Archean and Proterozoic systems (compare Figs. 38 and 90). In 

 some places, the exposed Cambrian borders the exposed parts of these 

 older systems on one side only, while in others it completely surrounds 

 them. Thus in the Black Hills of South Dakota, and in the Bighorn 

 mountains of Wyoming and Montana (Fig. 94), the Cambrian forma- 

 tions, as now exposed, completely encircle areas of pre-Cambrian rock. 

 In New York, Wisconsin, Texas, and British Columbia the Cambrian 

 beds nearly encircle areas of older rocks, while in various other places, 

 the Cambrian appears in linear belts along the sides of areas where the 

 rock is of greater age. This distribution is not peculiar to the Cambrian, 

 but is characteristic of most formations as compared with those of 

 greater age. 



The areas of Cambrian rock in the Appalachian mountains l are in 

 contrast with the foregoing. Here the exposed Cambrian formations 

 are not confined to the immediate borders of pre-Cambrian rocks, but 

 occur in parallel or sub-parallel belts (Fig. 95). This is the result of (1) 

 the folding to which the Cambrian and later strata of this region have 

 been subject, and (2) the erosion which the folds have suffered. Fig. 103 

 will help to explain the repetition of outcrops. In this diagram, A 

 represents pre-Cambrian strata, ~€ represents the Cambrian, and 0, 

 S, D, and C the Ordovician, Silurian, Devonian, and Carboniferous 

 systems, respectively. After strata are folded, erosion may cut them 

 down to any extent. The truncation of a fold below the level of the 

 Cambrian leads to the exposure of two belts of that system, one on 

 either side of a pre-Cambrian axis, as represented in the left-hand part 

 of the figure. If the truncation was at a level below the top, and above 

 the bottom of the Cambrian (right-hand side of Fig. 103), the strata of 

 that system are exposed in a single belt along the crest of the fold. If 

 parallel folds of unequal height occur, their truncation might give rise 

 to two belts of surface Cambrian in some cases, and in others to but one, 

 as illustrated by the figure. 



1 For details concerning the Cambrian system of the Appalachians; see the fol- 

 lowing folios of the U. S. Geol. Surv. : Gadsden, Ala. (Hayes); Ringgold, Ga.-Tenn. 

 (Hayes); Estilleville, Ky.-Va.-Tenn. (Campbell); Briceville, Tenn. (Keith); Chat- 

 tanooga, Tenn. (Hayes) ; Cleveland, Tenn. (Hayes) ; Kingston, Tenn. (Hayes) ; Knox- 

 ville, Tenn.-N. C. (Keith); Maynardville, Tenn. (Keith); Morristown, Tenn. (Keith); 

 Pikeville, Tenn. (Hayes) ; Bristol, Va.-Tenn. (Campbell) ; Harper's Ferry, Va.-Md.- 

 W.Va. (Keith); Monterey, Va.-W. Va. (Darton); Pocahontas, Va.-W. Va. (CampbeU)j 

 Staunton and Tazewell, Va.-W. Va. (Campbell). 



