246 GEOLOGY. 



with one fauna elsewhere, where the causes of biological change were notably 

 less. Between these interpretations, the former now seems the more probable. 



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, and hence of progressive 

 base -leveling. While at all stages there was doubtless some accumu- 

 lation of sediment on land- areas and in land- waters, the chief sedi- 

 mentation was in the sea, and the area of sedimentation may be regarded 

 as corresponding approximately to the area of the sea. In general, 

 the rate of sedimentation was probably faster in the early stages of 

 the period when the land-area was largest and probably highest, and 

 slower in the later stages after the land surface had been lowered by 

 erosion, and narrowed by the encroachment of the sea. Geographically, 

 the rate was probably more rapid near the land, and less rapid farther 

 from the shores in deeper water, in accordance with general law; but 

 possible exceptions to this are to be recognized at those stages when 

 very extensive and shallow epicontinental seas had been developed 

 by gradational processes, continental spreading, etc., for in such seas 

 deposition far from shore perhaps kept close pace with that near shore, 

 and may even in some cases have exceeded it. 



Sources and kinds of sediments. — As in other geologic periods, 

 the land-derived sediments came from the various formations then 

 exposed to erosion, the larger part from those which were undergoing 

 the most rapid degradation. The sediments gathered along the im- 

 mediate borders of the land were generally different from those being 

 deposited farther out at sea, but this difference was less marked as 

 broad shallow epicontinental seas were developed, for in such seas 

 the conditions were more nearly equal over wide areas. The deposits 

 in the abysmal depths of the ocean were of course notably different 

 from those of the terrigenous belt, but none of the former are known to 

 be now exposed. Even along shore there were considerable variations, 

 both because of variations in the character of the formations giving 

 rise to the sediments, and because of the differences in wave, river, 

 and current action. 



The Cambrian formations include all common phases of sedimentary 

 rocks. There are conglomerates, presumably accumulated near the 



