THE CAMBRIAN PERIOD. 257 



a much narrower outcrop. In general, the width of outcrop for a 

 formation of given thickness, so far as determined by topography, 

 depends on the angle between the bedding-planes and the surface where 

 the formation outcrops. The width of the outcrop decreases as this 

 angle increases. 



In an area such as Wisconsin, it is not to be supposed that the edge 

 of the Cambrian, as it laps up on older formations, represents the most 

 advanced shore-line of the Cambrian sea. The sea doubtless extended 

 farther up on the older land, depositing sands and other sediments on 

 its surface; but in the long periods of erosion which have ensued, the 

 edge of the formation as originally deposited has been destroyed (Fig. 

 101). Except in those cases where strata are absolutely vertical or 

 horizontal (and few strata occupy either of these positions), the low- 



Fig. 107. — Diagram illustrating the effect of topography on the width of the outcrop. 



ering of the surface by erosion must always shift the position of the 

 outcrop of any formation, and almost always in its direction of dip. 



Thickness. — Reference has already been made to the thickness of 

 the Cambrian system in some parts of the continent, and the relative 

 thickness in different portions of the continent is graphically repre- 

 sented by the sections of Fig. 96. These sections show the thickness to 

 be great in the mountain regions of both the east and west. In the 

 former region, the system attains a maximum known thickness of about 

 12,000 feet (more than two miles), and in the latter of something like 

 40,000 feet * (west of the Gold ranges in British Columbia), while over the 

 interior it rarely exceeds 1000 feet. The greater thicknesses were 

 probably accumulated near the shores of relatively high lands, where 

 the supply of sediment was great. 



Infirm Deductions from Thickness. 



The literature of geology is by no means free from infirm deductions 

 based on the reputed thicknesses of sediments. For example, when 

 10,000 feet of beds are assigned to the Cambrian section west of the 

 1 Dawson, Bull. Geol. Soc. Am., Vol. XII, pp. 62 and 64-§, 



