GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



373 



extends along it unbroken, with the Lowville beneath. Hence 



the Black Raver sea surrounded the region on all three sides 



with apparently unbroken connections, much diminishing the 



size of the former land areas of the region, even that of Beek- 



mantown times, which was the smallest of those that preceded 



it. The present outcrops of the Mohawk valley are near the 



old shore line, and the irregular, ridgy character of the bottom 



was the cause of the variations in thickness of the formation 



i 



there. Had erosion cut somewhat deeper, in other words, were 

 the exposures of the formation on a line somewhat south of 

 the present, it would undoubtedly extend east and west un- 

 broken. The Beekmantown pebbles in the Black River, in the 

 Tribes Hill-Amsterdam region, reported by Vanuxem and by 

 Prosser, are very significant as showing the near vicinity of 

 the shore line. 1 



Trenton formation. The Trenton formation may be said to 

 show a general uniformity in lithologic character all about the 

 Adirondack region, though with much variation in detail from 

 place to place. Instead of the quite pure, massive limestones 

 of the Chazy and Lowville formations, the major portion of the 

 Trenton is found to consist of thin bedded, black, shaly lime- 

 stones, often with partings of black, calcareous shales, the en- 

 tire formation being thus contaminated with a certain amount 

 of land wash in the shape of fine mud. The limestones are 

 usually hard and brittle, w T ith conchoidal fracture, though be- 

 coming thin bedded and shaly, and even the heavier beds split 

 thinly on weathering. 



In all sections there is considerable gray, rather coarsely 

 crystalline, fairly pure, very fossiliferous limestone, usually 

 thin bedded though sometimes becoming fairly massive. While 

 sometimes fairly persistent for considerable distances, such 

 beds are usually lens-shaped masses of restricted lateral extent, 

 entirely surrounded by the black calcareous muds of the ordinary 

 character. Beds of this sort seem less local and more persistent 

 in the Mohawk than in the Champlain valley, as has been pointed 

 out by White, and constitute a larger portion of the formation 

 in the former situation, indicating less local variation in the 



1 Geol. N. Y. 3d Dist. p. 44; 15th An. Rep't State Geol. 1 : 653. 



