THE CAMBRIAN PERIOD. 263 



Applying these considerations to the varying thicknesses of the 

 Cambrian beds, it is to be observed that any deep portion of the seas, 

 not simply the ocean border, which was filled from the side by a gradual 

 building out on the slopes, was liable to develop a thickness of beds 

 disproportional to the depth of the water. At the same time, the 

 superposition of great sheets of sediments covering broad epiconti- 

 nental areas, as those of the interior in late Cambrian time, was probably 

 rendered possible only by a change in the relation between the sea- 

 bottom and the sea-surface. This change probably consisted in part 

 in a raising of the sea-surface by sea-filling, and in part in the sinking 

 of the surface on which the sediments were deposited. 



The apportionment of the relative influence of these several factors 

 in the development of the observed thicknesses of rock is a difficult 

 matter, requiring a careful study of all available sources of evidence to 

 be found in the character of the deposits, in their relations to over- 

 lying and underlying formations, and in the topography of the Cam- 

 brian sea-bottom, so far as it can be reconstructed. Unless the 

 interpretation can be made with great assurance of correctness, in- 

 ferences as to crustal movements based simply on the thicknesses of 

 deposits may well be entertained with reserve, and doctrines based 

 on extreme subsidence inferred from these thicknesses may well be 

 regarded as lacking a firm basis. 



Selected Sections. 



The following sections of the Cambrian, at widely separated points, give some 

 idea of the range of the system both as to thickness and kinds of rock, and there- 

 fore of the variations in the conditions and in the rate of sedimentation. 



Section in Southeastern Newfoundland. 1 





Thickness in Feet. 



Characteristics. 





476 



Brown and black micaceous shales, with gray micaceous 



u 





sandstones; Bell island, Concepcion bay. 



a 



1426 



Red and green sandstones and slates, with calcareous beds at 



£§ 





the base. 





720 



Kelly's Island sandstones and shales, containing a few 



g-fj 





fucoids. 



.2 § 



150 



Black slate or shale; Middle Cambrian fossils. 



"O 



5 



Dark-gray limestone. 



§ 



1045 



Red, green, and black slates or shales; Middle Cambrian fos- 





. 



sils. 



1 Walcott (section after Murray), Bull. 81, U. S. Geol. Surv., pp. 257-8. 



