Handbook of Paleontology 303 



a waterlime containing a remarkable assemblage of 

 fossils, the eurypterids. The interior sea throughout 

 Salina age had been growing more shallow and finally 

 became land and remained so for a time. The Silurian 

 waters persisted in the Appalachian geosyncline (and 

 elsewhere) and there were deposited the limestones 

 of the closing period of the Upper Silurian (Cayugan) 

 epoch, the Cobleskill, Rondout and Manlius. 



The Silurian was largely a period of limestone build- 

 ing except in eastern United States which is charac- 

 terized, particularly in the lower part, by conglomer- 

 ates, sandstones and shales derived from Appalachia to 

 the east and carried by streams given added impetus 

 and carrying power by the uplift at the close of the 

 Ordovician. These sandstones increase in thickness 

 going east until they assume the character of great 

 sand deltas (Tuscarora of Pennsylvania; Shawangunk 

 of New York). The thickest accumulations are found 

 in east central Pennsylvania where a maximum thick- 

 ness of several thousand feet occurs. The lower part 

 of these deposits is almost devoid of limestones and 

 the upper part, constituting the Upper Silurian, con- 

 sists of shales gradually becoming more and more cal- 

 careous. Both in Lower and Middle Silurian the Ap- 

 palachian trough was plentifully supplied with sands 

 and muds, and the sand deltas show no fossils except 

 the worm trail Arthrophycus, showing that the trough 

 was not occupied by normal sea water. Fossils oc- 

 cur in the higher mud deposits when submergence per- 

 mitted free entrance of the sea to this part of the 

 trough. In the interior sea the waters were warm and 

 pure. Pure and magnesian limestones are found almost 

 entirely, particularly in the Niagaran, due to the abun- 



