Local Geology. 



FRANKLIN COUNTY. 

 Belmont. 



In the extreme north-western portion of Ellenburgh township, Clin- 

 ton county, is a low ridge of gneiss whose edge reaches further north 

 than any other exposure of the New York Pre-cambrian rocks, the small out- 

 lier at Burke excepted. It is flanked on the north and west by most excellent 

 exposures of basal Potsdam, consisting mainly of massive arkose conglomer- 

 ates, which reach over the border into Franklin county. Thence westward 

 the level drops sharply into the Chateaugay valley, which is heavily drift- 

 filled on its eastern side, so that no rock shows, and the boundary here is 

 uncertain. Like so many of the Adirondack valleys, the river here seems to 

 occupy a fault line. On the west the river hugs the side of a ponderous 

 ridge of gneiss which extends northward to the town line. Somewhat more than 

 a mile further down stream commences the series of excellent Potsdam exposures 

 which culminate in the " Chasm " at Chateaugay village. Still further down 

 stream the overlying Calciferous shows near the mouth of Marble river. The 

 section here has been measured and described by Mr. Walcott.* It shows 

 the upper 250' of the Potsdam, but gives no notion of the entire thickness of 

 the section along this line. 



Passing westward through Belmont the gneisses come up to the boundary 

 in a series of low ridges separated by shallow depressions tilled with drift. 

 The gneisses consist here of the red, poorly foliated, microperthitic variety, 

 alternating with dark grey plagioclase-pyroxene gneisses, in which the feldspar 

 is oligoclase or andesine and the pyroxene the aegerine-augite variety. The 

 former predominate toward the east and the latter to the west, but the two 

 occur interbanded in every section. There are also the usual dike-like bands 

 of gabbroic and dioritic gneiss. With the exception of the exposures in the 

 Chateaugay river but one outcrop of Potsdam sandstone was seen in the town- 

 ship in the vicinity of the gneisses. The exposure occurs in a depression 

 between two ridges of gneiss, and shows several layers of coarse, somewhat 

 pebbly rock, which is very quartzose and of light-brown color. Numerous 

 loose blocks occur wide-spread in the vicinity, some of which are of very 



• Bull. U. S. G. S. No. 81, p. 34:1. 



