REPORT OF THE STATE ^PALEONTOLOGIST 1902 1157 



Rondout formation 



The mingling of the Atlantic waters with those of the Missis- 

 sippian sea was of but short duration, and the Cobleskill was 

 brought to a close by the formation of a barrier in western New 

 York. This barrier as it grew caused a retrogression of the suc- 

 ceeding deposits lasting into Devonic time and effectually sepa- 

 rated the waters of the Atlantic from the Mississippian sea till 

 late Oriskany time, when some of the Oriskany sand was de- 

 posited in the crevices forming dikes and on the eroded surface 

 of the Cobleskill. 



With the elevation of the Cobleskill in western New York, the 

 true marine conditions under which the Cobleskill fauna flour- 

 ished were also brought to a close, and in New York there was 

 again formed a lagoon or bay in which the Kondout sediments 

 were deposited. This bay was similar in some respects to the 

 early Salina sea, but was diametrically opposite in that it formed 

 an arm of the Atlantic instead of the Mississippian sea. The 

 conditions following the close of the Cobleskill were such that 

 but few of the Cobleskill species pass up into the Rondout. That 

 the Rondout was, unlike the Cobleskill, a nonniarine formation 

 is shown by the return of the Eurypterus fauna, though much 

 reduced in force. At Seneca Falls Eurypterus have been found in 

 the Rondout but 2 feet below the Oriskany sandstone. They have 

 also been observed in several localities about Union Springs and 

 eastward. The writer has found Eurypterus remipes, 

 in place near Cherry Valley at a horizon which may possibly 

 prove to be the Rondout ; and, till the exact horizon is determined, 

 this find is chiefly interesting in that it is the most eastern 

 locality in New York from which Eurypterus has been obtained. 



In eastern New Y T ork and the Helderberg the conditions favor- 

 able for the existence of Eurypterus seem to have been lacking, 

 at least none have been found indicating that these localities 

 were in more direct communication with the sea. The fauna 

 found at the base of the Rondout in the Helderberg sections is 

 made up of species which continued their existence from the 

 Cobleskill, and which do not pass upward into the Manlius lime- 

 stone. In central New York there is a relation between the 



