﻿GEOLOGY OF THOUSAND ISLANDS REGION 8 1 



the southern branch of Horse creek on the Clayton quadrangle 

 and best along the Black river just east of the boundary of the 

 map. Such beds are seen in the lower third of the exposure on 

 plate 19; other bunchy surfaced layers also appear, with the 

 depressions filled in with shaly material, which seem clearly due 

 to rill action on tide flats. 



While the sand grains which are found in greater or smaller 

 number floating in the basal limestones indicate, if we may 

 follow recent investigations, -the conditions of quiet embayments, 

 in which sands washed in from the land, drifted out into the bay 

 and gradually sank to the bottom, becoming imbedded in the 

 limestone mud, the following beds indicate that this sea became 

 gradually deepened. The lower division still exhibits in the 

 shaly beds the sun cracks and ripple-marks and numerous mud 

 balls characteristic of mud flats while the upper beds in their 

 more uniform, massive character contain the criteria of deposi- 

 tion farther off the coast line. It follows thence that the Low- 

 ville sea was an advancing sea in the area here mapped. From 

 the development of the Lowville in the Mohawk valley and north 

 of the Adirondacks, it can be inferred that this transgression took 

 place from the southwest. In the Mohawk valley the distribution 

 of the Lowville is very erratic, as fully discussed by Cushing in a 

 former paper [Geology of the Northern Adirondack Region], it 

 being entirely absent in some localities while in others it is con- 

 nected by so called passage beds with the underlying Beekmantown. 

 This erratic distribution is then clearly due to the irregularity of 

 the surface over which the sea advanced, the Mohawk valley inter- 

 secting the deeply indented coast line of the Adirondack peninsula 

 in Lowville time. In the Champlain basin at the base of the Black 

 River group an outcrop of typical Lowville rock occurs in the Crown 

 Point section. The bed referred to consists of 5 feet of dove lime- 

 stone with Phytopsis tubes but otherwise apparently unfossiliferous. 

 However, 12 feet above this dove limestone the writer found 

 a large colony of Tetradium cellulosum together with 

 Ortho'ceras r e c t i c a m e r a t u m ,' another typical Low- 

 ville fossil, thereby clearly demonstrating the presence of the Low- 

 ville fauna in the Champlain basin. 



Four species of fossils have to be considered as highly charac- 

 teristic of the Lowville formation in the area here mapped, viz : 



Tetradium cellulosum (Hall) 



Orthoceras multicameratum (Emmons) Hall 



O. recticameratum Hall 



Bathyurus exlans (Hall) 



