THE CAMBRIAN PERIOD 405 



formations as shown on geological maps, gives a clue to the extent 

 of the continents and the positions of the shallow seas (epiconti- 

 nental; Greek, epi, upon) which at various times in the past covered 

 large areas of what is now land. These maps, showing the distri- 

 bution of the land and water in ancient periods, must be considered 

 as mere approximations, since (1) the absence of strata does not al- 

 ways prove the absence of seas in the past in any particular region, 

 because if the strata had been laid down they 

 might have been subsequently carried away , H I 



by erosion. For example (Fig. 375), 80 miles 

 from the nearest rocks of a certain age (De 



vonian) in Illinois, fossils of this age were found ,- IG 375 ' // evoni * n 

 ' . ' & sediments, A, round in 



in a fissure in rocks of older age (Silurian), the fissures of Silurian lime- 

 strata of the former having been entirely stone. This is the prin- 



eroded away. If this accidental discovery cipa ev * er * ce a • 



J J vonian strata at one time 



had not been made, there would have been covered an area in Illinois. 

 doubt as to the extension of the seas in De- 

 vonian times. Also, in the buried extensions of strata there may be 

 many interruptions where islands and peninsulas formerly existed. 

 (2) Much of the strata is often buried deeply under younger forma- 

 tions, and its distribution in such regions is uncertain. 



Basal Unconformity. — The lower layers of the Cambrian forma- 

 tions usually rest upon the eroded surface of older rocks, showing 

 that at the close of the Pre-Cambrian the continent of North America 

 was probably even larger than at present. The comparative levelness 

 of the Pre-Cambrian surface, except where it has been deformed by 

 later movements, indicates that erosion had been active and that 

 the land had been reduced to a comparatively level plain (peneplain, 

 p. 114). Upon such a surface the sea appears to have gradually en- 

 croached. The reason for the spread of the water may be found either 

 (1) in the actual sinking of the land or (2) in the raising of the sea 

 level in an amount equal to the volume of the sediments which were 

 being carried into the sea, displacing the water and causing it to over- 

 flow the land. As the sea encroached upon the land, it left upon its 

 ancient surfaces the coarse gravels and sands composed of fragments 

 of the older rocks, which occur at the base of the Cambrian system and 

 constitute the " basal conglomerate." 



Physical Geography of the Cambrian. — On evidence such as that 

 already mentioned (p. 403), it has been found that at the beginning 

 of the Cambrian (Fig. 373) the continent of North America was much 



