THE CAMBRIAN PERIOD 333 



that the sea advanced gradually and almost simultaneously 

 over central Europe and eastern Asia, as well as North Amer- 

 ica. Since continents can hardly be supposed to subside 

 evenly over so large a portion of the globe, the facts in this 

 case suggest a general rise of the ocean waters. The very 

 sediments which were being carried into the sea all through 

 the Cambrian period would inevitably displace a considerable 

 amount of water, and raise the sea level correspondingly. 



Cambrian strata differ according to locality. The Cam- 

 brian rocks are by no means alike in all localities, for the con- 

 ditions of sedimentation varied from place to place. Where 

 the sea advanced over a low shelving surface its waves and 

 currents reworked the soils and alluvial deposits already pre- 

 pared by weathering and wash, and sifted from them an 

 abundance of sand which was spread widely along the shores. 

 This may be the explanation of the very widespread Middle 

 and Upper Cambrian sandstone which represents the system 

 wherever it is exposed in the interior of the United States. 

 In the West, and in the Appalachian Mountains, the deposits 

 of Upper Cambrian age are principally limestones and shales, 

 indicating that in those districts conditions for clastic sedi- 

 mentation along shore had passed. During much of the 

 period the water may have been too deep to receive the coarser 

 sediments, but it is more probable that the lands were low 

 and remote, and were for that reason unable to furnish much 

 debris. Owing to the differences in the conditions of sedi- 

 mentation and in the time it continued, the total thickness 

 of the Cambrian strata is in some places great and in*others 

 small. Over the flat interior region the sea was apparently 

 shallow and came in late in the period, so that the sandy for- 

 mation (Potsdam sandstone) then produced is rarely more 

 than one thousand feet thick. In some places, on the other 

 hand, as in the Appalachian Mountains and in Nevada, 

 deposition of varying sediments seems to have continued 

 nearly or quite throughout the period, and to have resulted 

 in a succession of strata several thousand feet in depth. 



B. & B. GEOL. 19 



