Handbook of Paleontology 251 



by a series of shales and sands (Deepkill) of the same 

 age, characterized by graptolites belonging to the At- 

 lantic fauna and indicating a connection at that time 

 with the Atlantic ocean. The Levis beds at Levis op^ 

 posite Quebec are also such graptolite shales. The 

 Canadian (Beekmantown) and the Lower Ordovician 

 (Chazy) are represented by a great thickness of lime- 

 stone in northern and western Newfoundland and in 

 northwestern Scotland, and they have essentially the 

 fauna found in those rocks in our Champlain valley 

 showing that these areas belonged to the same marine 

 province in these periods. 



The Ordovician system is separated from the Cana- 

 dian by a complete break throughout North America 

 due to a withdrawal of the sea at the end of Canadian 

 time. After its return, at several times during the 

 course of the Ordovician period the retreat of the ma- 

 rine waters was so nearly complete that if any re- 

 mained it could be only in certain of the now 

 deeply covered basins. During this period and the 

 preceding North America was little above sea level, 

 and uplands occurred only along the margins of the 

 continent. These periods, therefore, were character- 

 ized to a large extent by limestone building since the 

 seas were receiving less sediments because of the low 

 relief of the surrounding land. The transgressions of 

 the sea during the Ordovician were extensive (Low- 

 ville and Trenton seas), but less extensive, certainly, 

 than the Upper Ozarkian sea and probably also less 

 widely spread than two of the Canadian stages (Ul- 

 rich). The down-warping of the eastern interior of 

 the continent permitted the entrance of seas, not con- 

 nected with the Atlantic ocean, to cover large areas 



