FORMATIONS AND PHYSICAL HISTORY 349 



Cambrian and with Acadian (p. 344). In the western part of North 

 America, and on the opposite side of the North Pacific as well, the 

 Middle Cambrian does not contain Paradoxides. Its fauna is 

 known as the Olenoides fauna, which is distinct from fauna of the 

 Lower Cambrian. In like manner the Middle Cambrian fauna 

 is succeeded by another, the Dikellocephalus fauna found in the 

 Upper Cambrian strata. Geologists have agreed to define the 

 Upper Cambrian as the series of strata carrying this fauna. 



It is not to be understood that every species of the Paradoxides 

 fauna is unlike every species of the faunas below and above. This 

 is not the case; but so many species of the three faunas are different, 

 that with a considerable number to judge from, their separation is 

 possible by those familiar with Cambrian fossils. 



Sequence of faunas based on stratigraphy. The sequence of 

 faunas was first determined by superposition of the strata. The 

 Lower Cambrian fauna could not have been known to be older than 

 the Middle Cambrian fauna if beds containing the former did not 

 underlie beds containing the latter. In other words, the primary 

 basis for correlation by means of fossils is stratigraphy. 



Physical Events of the Cambrian 



Submergence. The distribution of the several series of the 

 system shows that the great physical event of the Cambrian period in 

 North America was progressive submergence of the continent. Theo- 

 rectically, this may have been brought about by a rise of the sea or 

 by a lowering of the land, or by both together. Both the lowering 

 of the land and the rise of the sea may be due to gradation, to dias- 

 trophism, or to the two combined. 



Gradation as a cause of submergence. Gradation is perpetual 

 and inevitable where land and sea exist. The waves attack the 

 land along its borders, and the agents of land degradation lower 

 its surface. The former is a direct cause of encroachment of sea 

 on land, and the latter is an indirect cause, since all sediments car- 

 ried from land to sea raise the surface of the sea correspondingly. 

 Small as this rise is for any brief period, its effect is to cause the sea 

 to advance on the land, and the lowering of the land by degradation 

 at the same time increases the area of the advance. // continued 

 long enough, shore-cutting about the borders of the lands, down- 

 cutting over the whole surface, and the accompanying rise of the 

 sea-level, must inevitably cause the water to cover the continents 



