256 



GEOLOGY. 



Width of outcrops. — Certain other principles of geological cartog- 

 raphy are illustrated by the outcrops of the Cambrian system. The 

 most extensive continuous outcrops (Fig. 95) are in Wisconsin, and 

 yet in that State the Upper Cambrian only, with a thickness of less than 

 1000 feet (see Fig. 96) is present, while in the Appalachian mountains^ 

 where the Upper, Middle, and Lower Cambrian are all present, with an 

 aggregate thickness of several thousand feet, the system appears at the 

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

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

 From Fig. 95 it is also seen that in various parts of Nevada, Utah, 

 Montana, and British Columbia, where the system is many times as 

 thick as in Wisconsin, the outcrops of the system are much narrower. 



The explanation of this apparent anomaly is to be found primarily 

 in the attitude of the strata. In Wisconsin they are nearly horizontal, 

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

 at high angles. Where the strata are vertical or nearly so, the width of 

 their outcrop on a horizontal surface is about the same as the thickness 

 of the beds, as shown in Fig. 103 and the right-hand side of Fig. 106. 

 Where they are nearly horizontal, as in the left-hand side of Fig. 106 



Fig. 106. — 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. 



the width of the outcrop is much greater. Thus in the Appalachian 

 mountains, where the thickness of the strata is locally as much as two 

 miles, their high inclination reduces their outcrop to a narrow belt, 

 while the corresponding strata in Wisconsin, less than 1000 feet thick, 

 but nearly horizontal, have a much wider outcrop. 



It is not to be inferred, however, that horizontal strata necessarily 

 determine a wide outcrop. The width of the outcrop is also influenced 

 by topography, as shown in Fig. 107. Here the horizontal stratum 

 between B and C has about the same thickness as C of Fig. 106, but 



