406 HISTORICAL GEOLOGY 



expanded, the Atlantic shore being farther east than now. On the 

 east a narrow sea stretched from Alabama northeast to Labrador, 

 separated from the ocean by a land of unknown eastern extent called 

 Appalachia, but whose western shore line was drawn near the site 

 of the present Blue Ridge. In the west a similar sea existed which, 

 at its greatest extent, reached from California to the Arctic Ocean. 



The submergence of the continent continued in the Middle Cam- 

 brian, at which time a portion of the central United States was covered 

 by seas whose shallowness is shown by ripple marks in the sandstone, 

 and even by sun cracks made by the drying out of sediments exposed 

 to the sun's heat. In the Upper Cambrian (Fig. 374) the seas spread 

 over portions of the continent which were land in the Middle Cam- 

 brian and were withdrawn from others which had been covered by 

 the Middle Cambrian seas, and in still other portions the sedimenta- 

 tion continued, showing that the seas remained as before. Conse- 

 quently near the close of the Cambrian the physical geography was 

 very different from that in the early epoch, the water covering a much 

 larger area than in the latter. 



Character of the Cambrian Rocks. — The Cambrian formations are 

 composed of sedimentary rocks which vary in character from place 

 to place. Where the sea advanced over a low shore in which there 

 was an abundance of soil or other loose material, the waves and cur- 

 rents worked them over and spread them upon the sea bottom. Such 

 was doubtless the origin of the Middle and Upper Cambrian sand- 

 stones which are so widespread in the interior of the United States. 

 The occurrence of limestones and shales in the West and in the 

 Appalachian Mountains indicates either that the shores were distant 

 in these regions, or were so low that the gradients of the streams were 

 insufficient to permit the latter to move any but fine material and such 

 salts as were in solution. 



The thickness of the formations of the period varies from a few 

 hundred to twelve thousand feet. This variation is due to the fact 

 (1) that in some places deposition took place longer than in others, 

 and (2) that in other places where erosion was rapid and the condi- 

 tions favorable to sedimentation, the ocean bottom was built up 

 rapidly by the sand and gravel brought in by the streams and waves. 

 (3) In other regions, where the land was low, or (4) in portions of the 

 seas distant from the shore, the sedimentation may have taken place 

 with extreme slowness, so that in thousands of years the thickness 

 ol sediment accumulated was a small fraction of that laid down in an 



