370 THE CAMBRIAN PERIOD 



Ridge, extended eastward an unknown distance into the Atlantic. 

 On the western shore of the Appalachian land was a narrow arm 

 of the sea, which opened both to the north and south and sepa- 

 rated this land area from the great mass of the continent. The 

 site of the Sierra Nevada was occupied by a long, narrow land, 

 running from Puget Sound to Mexico, and another such area was 

 found in eastern British Columbia. The Great Basin region was 

 under water. Around these shores were laid down the coarser 

 deposits of the Lower Cambrian, with great masses of shales and 

 some limestone in deeper water. 



In the course of time the continent was slowly depressed, the sea 

 gradually advancing from the south during the Middle Cambrian, 

 and reaching its greatest extension in the Upper. Toward the 

 close of the period a large part of the continent had been sub- 

 merged and, in particular, a vast interior sea had been established 

 over the Mississippi valley. 



Cambrian in Other Continents. — In Europe the Cambrian 

 rocks are even more fully developed than in North America, 

 having in Wales a thickness of 20,000 feet of conglomerates, sand- 

 stones, shales, slates, and quartzites. These rocks bear witness to 

 their shallow-water origin and were deposited on a slowly sinking 

 sea-bottom. The Cambrian rocks have their maximum thickness 

 along the western side of the continent, being four times as thick 

 in Wales and Spain as in Germany and Bohemia. In Russia the 

 rocks of this period are remarkable for their unconsolidated 

 character ; at the base are 300 feet of plastic clays, which look 

 as though they had just been abandoned by the sea. In central 

 Russia the Cambrian dies out and the Ordovician strata rest 

 directly upon the Archaean. Cambrian rocks occur extensively 

 in northeastern China, in India, in Australia, and in the Argentine 

 Republic. 



Cambrian Life 



The Cambrian fauna is of extraordinary interest, because it is 

 the most ancient that we know, but the most superficial examina- 

 tion of it shows that it cannot represent the beginnings of life 



