THE CAMBRIAN PERIOD 



403 



the border of the Pre-Cambrian shield (p. 389) and the Pre-Cambrian 

 mass of the Adirondack^, and in regions where the Cambrian has been 

 exposed by the deep erosion of regions which have been raised and 

 folded, as in the folded Appalachians, from the St. Lawrence to Ala- 

 bama, and in portions of the West. For the most part, however, 

 Cambrian rocks in 

 North America, al- 

 though of wide ex- 

 tent, are not exposed 

 at the surface over 

 large areas, being 

 deeply buried under 

 younger strata. 



Physical Geog- 

 raphy of Ancient 

 Periods. — The de- 

 termination of the 

 distribution of land 

 and water in such 

 remote periods as 

 the Cambrian is very 

 difficult, and at best 

 the outlines of the 

 continents, oceans, 

 and seas are only ap- 

 proximately known. 

 Maps of the kind 

 shown here (Figs. 



373> 374) are based 

 upon several lines of 

 evidence. 



(1) When the fos- 

 sils of a formation of 

 known age are found to be of practically the same species in outcrops 

 that are widely separated, it is assumed that the waters in which 

 they lived were either connected by broad straits, or, if nothing 

 points to a different conclusion, that they inhabited the same seas. 

 If, however, they are found to differ widely in species in regions 

 which may, for example, be less than fifty miles apart, although 

 the conditions under which they lived were apparently the same, 



Fig. 373. — Map showing the probable distribution of 

 land and water in North America during Lower Cambrian 

 times. The shaded portion is land. The Lower Cambrian 

 sediments were laid down in long, narrow straits. (Modified 

 after Schuchert.) 



