THE CAMBRIAN PERIOD. 271 



Scandinavia, where Lower, Middle, and Upper Cambrian all are present, 

 the aggregate thickness is sometimes no more than 400 feet. In 

 western Russia also it is thin. These differences probably mean that 

 sediments were being deposited in some places many times as rapidly 

 as in others. 



The commonly accepted subdivisions of the British Cambrian are 

 (1) the Harleck and Llanberis group (Olenellus zone), (2) the Menevian 

 group (Paradoxides zone), and (3) the Lingula flags (Olenus zone). 

 The Tremadoc slates are sometimes classed as Upper Cambrian. 



The subdivisions of the Cambrian are notably unlike in different 

 parts of the continent. The faunal unlikenesses are such as to seem 

 to divide the continent into two natural provinces, a northern and a 

 southern. To the northern division belong the areas of Cambrian rock 

 found in Russia, Scandinavia, and north Scotland; to the southern, 

 those found in Bohemia, 1 France, Spain, Portugal, Sardinia. It is 

 commonly believed that some sort of barrier must have existed between 

 these provinces which prevented marine life from passing freely 

 between them. It has also been suggested that the differences in 

 faunas between the two regions may be due, at least in part, to climate; 2 

 but this suggestion finds little support so far as present knowledge goes 

 in other parts of the world. The fossils of the northern provinces of 

 Europe have much resemblance to those of the Atlantic border of 

 America, suggesting that geographic and climatic conditions were such 

 as to allow of the free migration of marine life from northern Europe 

 to eastern America, and vice versa. 



In Europe the Middle Cambrian is more wide-spread than the Lower 

 or Upper, 3 showing that changes in the relation of sea and land were in 

 progress during the Cambrian period, shifting the areas of erosion and 

 sedimentation. It is a matter of interest to note that the Cambrian 

 history of western Europe seems to have some correspondence with 

 that of eastern North America, while that of central and eastern Europe 

 has more likeness to that of the interior of our continent. 



The total area where the Cambrian rocks are exposed in Europe 

 is small, and the outcrops sustain the same general relations to older 

 formations as in North America. The actual extent of the system is 



1 Made famous by the classic studies of Barrande; Systeme Silurien de la Boheme. 



2 Kayser and Lake, Comparative Geology. 



3 Freeh, Neues Jahrbuch fur Mineraiogie, 1899, Bd. II, pp. 164-176. 



