THE CAMBRIAN PERIOD. 



253 



the areas of pre-Cambrian rock, is not its original border, and does not 

 represent the shore-line of the Cambrian sea when its waters were 

 most wide-spread. Fig. 101 represents the conditions which often 



Fig. 101. — Diagram illustrating the relation of Cambrian formations, £7, to older rocks. 

 The diagram suggests that the Cambrian formations have been eroded back from 

 the original margins; thus A once extended to A', B to B' , C to C". 



exist. Each of the Cambrian formations, represented by A, B, and G, 

 formerly extended farther to the left. 



The areas where the Cambrian formations are exposed are not to 

 be confounded with the areas where they actually exist. In Fig. 95 

 both these classes of areas within the area of North America are repre- 

 sented. The Cambrian formations are exposed, for example, in Wis- 

 consin, in Missouri, and in Texas; but the strata of Texas are doubtless 

 continuous beneath younger formations with those exposed in Missouri, 

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

 those of the Black Hills on the west, and with those of New York on 

 the east. The diagrammatic section shown in Fig. 102 suggests the 



Fig. 102. — Diagram illustrating the general relations of Cambrian beds in the interior. 

 The Cambrian, €, is represented as appearing at the extremes of the diagram, 

 and as dipping below younger beds between. 



relations which are believed to exist. By far the larger part of the 

 surface on which Cambrian formations were deposited was afterwards 

 covered by younger formations. In so far as these overlying formations 

 have not been removed, the Cambrian formations are still buried. 

 This is doubtless true to a greater extent for the ocean bottom than 

 for the continents. 



Position of outcrops. — The study of the maps (Figs. 90 and 95) 

 showing the areas where the formations of the Cambrian system are 

 now exposed reveals several points of significance. In the first place, 

 the outcrops of the formation occur for the most part in association with 



