GEOLOGY 



parallel belts (Fig. 358). This is the result of (a) the folding to 

 which the Cambrian and later strata of this region have been sub- 

 ject, and (b) the erosion which the folds have suffered. Fig. 362 

 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 Carbonif- 

 erous systems, respectively. After the strata were folded, erosion 

 cut the folds down. A fold which involves Cambrian beds, if trun- 

 cated below the level of the bottom of these beds at their highest 

 point, exposes two belts of Cambrian strata, one on either side of 

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

 figure. If the truncation is at a level below the top and above 

 the bottom of the Cambrian (right-hand side of Fig. 362), the strata 

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

 fold. (3) . In some places, Cambrian outcrops are surrounded by 

 older formations. In such cases the Cambrian outcrops presum- 

 ably represent remnants which have escaped erosion. They might 

 occupy depressions in the surface of pre-Cambrian formations, or 

 they might constitute hills (Fig. 363). 



Width of outcrops. The most extensive continuous outcrops 

 of the Cambrian (Fig. 358) are in Wisconsin; yet there the Upper 



Fig. 364. Diagram illustrating the influence of dip on the width of outcrop. 

 The Cambrian beds, &, to the left have a much wider outcrop than the 

 Cambrian beds to the right, though the thickness is the same. 



Cambrian only is present, with a thickness of less than 1,000 feet, 

 while in the Appalachian Mountains, where the system has an 

 aggregate thickness of several thousand feet, it appears at the 

 surface in narrow belts. That is, the outcrops are narrow in the 

 east where the system is thick, and wide in the interior where it is 

 thin. The explanation of this apparent anomaly is found primarily 

 in the attitude of the strata. In Wisconsin they are nearly hori- 

 zontal, while in the mountain regions, both east and west, they are 



