CAMBRIAN PERIOD. 513 



The last-mentioned group of rocks, namely, the Skiddaw 

 Slates of the north of England, is in a doubtful position. 

 They consist of about 7000 feet of dark-coloured shales and 

 slates, and they are most clearly the equivalent of the Quebec 

 group of Canada, containing many of the same fossils. Upon 

 the whole, it seems safer in the meanwhile to regard them as 

 Upper Cambrian. 



II. Cambrian Rocks of Bohemia and Sweden. In Bohemia, 

 M. Barrande has succeeded in demonstrating as underlying 

 the Lower Silurian rocks of that country a zone of rocks, 

 which correspond to the Lingula Flags of Britain, and are 

 therefore of Upper Cambrian age. This zone contains many 

 remarkable and characteristic fossils, and is often spoken of as 

 the " Primordial Zone." In Sweden and Norway the Lower 

 Cambrian rocks are represented by a sandstone containing 

 impressions supposed to be referable to sea-weeds or " fucoids." 

 This " Fucoidal sandstone " is succeeded by beds of so-called 

 " alum-schist/' which are of Upper Cambrian age, and corre- 

 spond with the Lingula Flags of Britain. 



III. Cambrian Rocks of North America. The Cambrian 

 rocks are represented in North America by the Potsdam sand- 

 stone and the Calciferous series. The Potsdam Sandstone is 

 mostly a laminated sandstone, or grit in the State of New 

 York, but limestones are present in addition in the Mississippi 

 basin, and it consists of a great thickness (2000 to 7000 feet) 

 of slates, sandstones, and limestones, along the Appalachian 

 chain. It contains a good many fossils, among which are 

 Trilobites resembling those of the " Primordial Zone " in 

 Bohemia. 



The Calciferous series consists of a hard calcareous sand- 

 stone, or " sand-rock " in the State of New York ; but it con- 

 sists of sandstone with well-developed magnesian limestone in 

 the basin of the Mississippi ; and along the Appalachian chain 

 it consists of sandstones and limestones, subordinated to great 

 masses of shale. In their last-mentioned development the 

 Calciferous rocks have been termed the " Quebec group," and, 

 as before said, they are undoubtedly the equivalent of the 

 Skiddaw Slates of Britain. They attain a thickness of from 

 5000 to 7000 feet ; but it is not clear whether they are truly 

 referable to the Upper Cambrian or to the base of the Silurian 

 system. Most probably they are transition beds between the 

 two formations. 



LIFE OF THE CAMBRIAN PERIOD. In the Lower Cambrian 

 Rocks fossils have hitherto proved extremely scarce. The 

 commonest organic remains are the burrows and tracks of 

 2 K 



