THE CAMBRIAN PERIOD. 



249 



in this region indicates that the deposits were made, in many cases, in 

 narrow arms of the sea, surrounded, or nearly surrounded, by high 

 lands composed of older formations. Limestone is, however, not 

 altogether wanting (Fig. 98). The fossils of this region indicate that 

 a land barrier existed to the east, in the region which is now covered 

 with water, shutting off the region where the known strata accumulated 

 from free connection with the open sea in this direction. They are 

 also thought to indicate that the waters in which the sediments of this 



Fig. 98. — Nodular limestone, Lower Cambrian, from the east side of Smith Point, 

 Smith Sound, Trinity Bay, Newfoundland, (Walcott, U. S. Geol. Surv.) 



region accumulated were not in free communication with those in which 

 the sediments of the Appalachian region were laid down. 



The fact that in the northern interior of the United States the 

 Upper Cambrian formation is generally of sandstone, and that this 

 sandstone is wide -spread, indicates that the water was so shallow during 

 its deposition that the waves were competent to transport sand for 

 long distances. Furthermore, the structure of the strata, with their 

 cross-bedding (Fig. 99) and ripple marks, shows that the whole series, 

 so far as seen, was deposited in shallow water, and therefore on a base 

 which was gradually submerged as the sediments were accumulated. 



