108 NEW YORK STATE MUSEUM 



stone iR^bbles, with their ptH-uliar faunal clcMuents, are quite dis- 

 tinct from the Treiilon exposed in tlu' Mohawk viilley, nor have 

 similar Trenton fossilfi been rcconied from more iiortlierly, or 

 southerly localities. 



The sum of the evidence poiuL^, Lli«\ninir, lo an origin of the 

 eonglomerate pebbles from a direction other than the west, or 

 to the area of Appalachian foldinj:: between the lower Mohawk 

 and Taconic mountains. 



The tectonic events of lower Siliiri( time have Im'cii described 



with a master's hand by Dana: 



The era of limestone-makinj; and ihereloic 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 i>eriod, 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 Icvtd at the 

 time, it would have be(>n 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 Ai)palachians, and some 

 change of level had therefore taken place. P»ut, as the formation 

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

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

 essentiallv these regions. If th<^ Atlantic hairier above alluded 

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

 ing it, and raising toward the surface another ].;ii;ill('l region 

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

 oiK^^ned again the New York and A].palachian area to the ocean, 

 and so might have occasioned ilw trnnsition to sedimentary 

 accumulations. 



The barrier, assumed l)y this profound student, to account for 

 the undisturbed deposition of the lower Lower Siluric up to the 



