THE CAMBRIAN PERIOD 



337 



FIG. 333. A large 

 trilobite (Olenellus) 

 characteristic of 

 the lower Cam- 

 brian rocks. 



Climate of the Cambrian period. In 



the days when the Laplacian or gaseous 



theory of the earth's origin was generally 



accepted as true, it was thought that, in a 



period so remote as the Cambrian, the at- 

 mosphere must have been distinctly warmer, 



more moist, and more heavily charged with 



carbon dioxide than now. There was no 



direct evidence, however, that such condi- 

 tions really existed, and in more recent 



years some facts have been discovered 



which effectually show that they did not. 



Glacial deposits of early Cambrian age 



exist in Norway, China, and probably 



elsewhere. In China the glaciers were not 



far from sea level in about the latitude of 



New Orleans. From this it is reasonable to 



infer that the general climate of the earth 

 in the Cambrian pe- 

 riod was not radically 

 different from that 

 which prevails at 

 present. 



Close of the period. 

 The Cambrian sys- 

 tem is somewhat ar- 

 bitrarily set off from the Ordovician 

 because of a difference in the fossils 

 which the rocks contain. It is probable 

 that, when the history of the two periods 

 is better known, a more rational means 

 of separation will be found. 



Estimates of the length of the Cam- 

 brian period. There is no satisfactory 

 means of determining the number of 

 years in any of the geologic periods. 



FIG. 334. A large 

 trilobite (Dikelo- 

 cephalus) charac- 

 teristic of the late 

 Cambrian rocks. 



FIG. 335. Supposed 

 pteropod shells (Hyo- 

 lithes) in a bit of 

 Cambrian shale. 



FIG. 336. A cap- 

 shaped gastropod 

 from the Cambrian 

 system (Stenotheca). 



