GEOLOGY AND PALEONTOLOGY OF THE SCHOHARIE VALLEY 131 



The sudden transition from Hudson sandstones to limestones 

 of Upper Silurie age, lias been frequently noticed by writers. The 

 absence of clastic silicious material at the base of the limestones 

 has been particularly commented on. Van Ingen and Clark 

 record the presence of boulders of sandstone in the " Coralline m 

 limestone near the contact with the upturned and strongly 

 eroded Hudson river sandstones at Rondout, but these boulders 

 are overgrown with Bryozoa and corals, showing that they did not 

 form a part of an actively eroded shore. Had the shore gradu- 

 ally advanced landward, there must of necessity have been formed 

 a deposit of silicious elastics, derived from the underlying beds. 

 In certain cases such an accumulation of clastic material actually 

 occurred as shown by the basal beds at New Salem. As a rule 

 however such beds are absent, indicating a sudden deepening, 

 so that offshore deposits could accumulate on the silicious 

 basement. 



This sudden advance of the seashore seems to have produced 

 a breach in the barrier which separated the Atlantic waters from 

 those of the interior continental or Mississippian sea. Through 

 this breach the Atlantic fauna found an entrance, so that in the 

 deposit of this time we have a mingled Atlantic and Mississippi 

 sea fauna. 2 



The fauna of the Cobleskill of Schoharie seems to be a com- 

 posite of the fauna of the lower Manlhis of the interior (Green- 

 field limestone fauna) and that of the Atlantic province of Silurie 

 time. We know from the Silurie deposits in the Atlantic province 

 that species which in the interior were confined to the Niagaran 

 beds continued in the open ocean practically throughout the 

 Silurie. With the invasion of the Atlantic waters, we would expect 

 these species to make their way into the interior basin again but to 



x Tbe Coralline limestone of Rondout lias born correlated by Hartnagel 

 witb tbe Wilbur limestone, a name applied by him to the basal bed of lime 

 sandrock found in many places between the Kosendale cement, and the 

 Binnewater sandstone. There is some reason to believe however thai tins 

 Rondout Coralline is the Cobleskill. whicn here rests directly on a knob of 

 Hudson, around which the Rosendale beds were deposited. [See p. 312] 



'-'See Hartnagel. loc. cit. p. 1155. 



