252 



GEOLOGY, 



Igneous Rocks. 



At a few points igneous rocks are known to be associated with the 

 stratified rocks of the Cambrian system. This is true in some parts of 

 Newfoundland and Massachusetts. Igneous rock is also associated 

 with some of the rocks thought to be Cambrian, lying west of the north- 

 ern continuation of the Green mountains. Considerable thicknesses 

 of igneous rock occur west of the Gold ranges in British Columbia. 1 





Fig. 100. — Figure showing inequalities of layers as shown by weathering, 

 sandstone, Dells of the Wisconsin. (Atwood.) 



Potsdam 



It is not always possible to tell whether igneous rocks associated with 

 the Cambrian strata were erupted in the Cambrian period or at some 

 later time. 



Distribution, Thickness, and Outcrops of the Cambrian System. 



The Cambrian formations were once as wide-spread as the Cambrian 

 seas themselves, but it does not follow that they now exist over all the 

 areas they once covered. Where exposed, the Cambrian rocks have 

 suffered erosion, and the border of the system, as it now appears about 



1 Dawson, Bui. Geol. Soc. Am., Vol. XII, p. 85. 



