FORMATIONS AND PHYSICAL HISTORY 351 



than this, rock in some mines is observed to creep. It is not im- 

 probable, therefore, that such a pressure, constantly exerted for 

 a prolonged period, might cause some spreading of the great con- 

 tinental platforms, and hence (i) some lowering of their surfaces, 

 (2) some submergence about their borders, and (3) at the same 

 time some rise of the sea-level. Many phenomena which cannot be 

 cited here seem to lend support to this hypothesis of lateral creep, 1 

 but its efficiency is not determined. 



Sedimentation in the Cambrian Period 



Sedimentation in the Cambrian period appears to have followed 

 the general laws that govern deposition in periods of comparative 

 freedom from great deforming movements. Most of the known 

 sediments were deposited in the sea, and their area may be regarded 

 as a rough measure of the area of the Cambrian sea. Sedimentation 

 was probably faster in the early stages of the period when the land- 

 arva was largest and highest, and slower in the later stages after 

 the land had been lowered and narrowed. Sedimentation was 

 probably greatest near the land. 



Sources and kinds of sediments. As in other periods, the land- 

 derived sediments came from all formations exposed to erosion. 

 The sediments along the immediate borders of the land were doubt- 

 less different from those farther out, and even along shore probably 

 there were variations, because of differences (i) in the sources of 

 the sediments, and (2) in wave, river, and current action. 



The Cambrian system includes all common phases of sedimen- 

 tary rocks. There are conglomerates, presumably laid down near 

 the shores of the time; sandstones, the sand of which was deposited 

 in shallow water; shales, representing the mud deposits in quiet 

 water; and beds of limestone representing, for the most part, the 

 accumulations of shells, etc., where sediments from the land were 

 not abundant. 



Geographic variations. The distribution of these various sorts 

 of sedimentary rocks shows that various kinds of detrital beds were 

 accumulating in different places at the same time, and at the same 

 place at different times. Not only this, but they were accumulated 

 at very different rates, as the great variations in thickness show. 



The fact that the Upper Cambrian in the northern interior of 

 the United States is mostly of sandstone, and that this sandstone is 



1 Chamberlin and Salisbury, Earth History, Vol. II. 



