186 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



erosion before the coarse sediments were laid down. The same horizon appears 

 to be represented in southeastern Alaska by argillites. The third epoch of deposi- 

 tion is one of calcareous material, and here again there was an abrupt change, this 

 time to deep-sea conditions and marked by the appearance of a new fauna. This 

 Upper Carboniferous sea was probably widespread. Its thickest recognized de- 

 posits are found in southeastern Alaska, but it seems quite probable that the 

 upper limestone member of the Cache Creek series was deposited in it. Sedi- 

 mentation may have continued unbroken into Triassic times in the Yukon 

 district, but in southeastern Alaska a period of erosion intervened between the 

 highest Paleozoic and the lowest Mesozoic.'" 



