108 NEW YORK STATE MUSEUM 



stone pebbles, with their peculiar faunal elements, are quite dis- 

 tinct from the Trenton exposed in the Mohawk valley, nor have 

 similar Trenton fossils been recorded from more northerly, or 

 southerly localities. 



The sum of the evidence points, therefore, to an origin of the 

 conglomerate pebbles from a direction other than the west, or 

 to the area of Appalachian folding between the lower Mohawk 

 and Taconic mountains. 



The tectonic events of lower Siluric time have been described 

 with a master's hand by Dana: 



The era of limestone-making and therefore of continental seas, 

 largely free from sediments, which made progress in the Cana- 

 dian period, reached its culmination in the earlier division of the 

 Trenton period, when limestones were almost the only kind of 

 rock being deposited over the breadth of the continent. The 

 absence of sediments from a large part of the continental region 

 must have been owing to the absence of the conditions on which 

 their distribution depends. The currents of the ocean which 

 ordinarily swept over the land (the Labrador currents from the 

 north, along the eastern borders, and the Gulf stream from the 

 south, over the interior) must have had their action partly sus- 

 pended. This may have been caused by a barrier outside of the 

 limestone area, near or outside of the present Atlantic coast 

 line. If the land in the shallow region outside of the present 

 Atlantic border of the continent, were above tide level at the 

 time, it would have been a continental barrier against both waves 

 and currents. 



With the opening of the Hudson river era, sediments again 

 were deposited over New York and the Appalachians, and some 

 change of level had therefore taken place. But, as the formation 

 of the limestones was continued in the Missisippi basin, and also 

 in the St Lawrence bay (at Anticosti), the change did not affect 

 essentially these regions. If the Atlantic barrier above alluded 

 to were a fact in the Trenton era, an oscillation of level submerg- 

 ing it, and raising toward the surface another parallel region 

 more to the west, where the Appalachians now stand, would have 

 opened again the New York and Appalachian area, to the ocean, 

 and so might have occasioned the transition to sedimentary 

 accumulations. 



The barrier, assumed by this profound student, to account for 

 the undisturbed deposition of the lower Lower Siluric up to the 



