THE CAMBRIAN PERIOD 61 



Cambrian is singularly free from igneous rocks and thus presents a 

 remarkable contrast with the preceding eras. 



Physical History 



Great Basal Unconformity. — We have already learned that a 

 profound and seemingly almost universal unconformity separates 

 the Archean and Algonkian rocks. Another great unconformity 

 separates the Algonkian and Paleozoic rocks. Cambrian strata 

 rarely, if ever fail to rest upon the eroded surfaces of either the 

 Archean or the Algonkian. C. D. Walcott has recently (1914) 

 stated that no definitely proved transition rocks between the 

 Cambrian and pre-Cambrian are known in North America. It 

 has been definitely proved, as for example in the Adirondack region, 

 to be quite the rule that the Cambrian sediments not only rest upon 

 an eroded surface of older rocks, but that the surface of these 

 latter had been worn down to the condition of a more or less well- 

 developed peneplain. Accordingly, just before and during early 

 Cambrian time, most of North America must have been dry land 

 suffering erosion. Conglomerates containing pebbles of the older 

 rocks are of very common occurrence at the base of the Cambrian 

 sediments. The great duration of this erosion interval which pro- 

 duced such a profound unconformity, not only in North America 

 but in other continents as well, is regarded as one of the greatest 

 physical events of its kind in the history of the earth since the 

 beginning of Paleozoic, or rather late Proterozoic, time. 



Early Cambrian. — "The great physical event of the Cam- 

 brian period in North America was the progressive submergence 

 of the continent " (Chamberlin and Salisbury) . As we have already 

 learned, such a submergence may have been produced either by 

 rising sea level, or subsidence of the land, or both. In the case of 

 the Cambrian submergence there appears to be no escape from the 

 conclusion that a rise of the sea was an important factor, since the 

 development of such an extensive peneplain surface implies that 

 the continent must have remained almost unaffected by diastrophic 

 movements for a long time, and the tremendous volume of material 

 removed and dumped into the sea must have very appreciably 

 raised its level. 



Wherever Lower Cambrian marine strata (actually exposed or 

 concealed) rest directly upon pre-Cambrian rocks we can be sure 



