548 



Report of the State Geologist. 



orthoclase in addition to the ordinary gabbro minerals, plagioclase, 

 hornblende, angite, garnet and apatite. It is the most westerly of the gabbro 

 outcrops, but two miles of barren ground intervene between it and the 

 Clintonville gneisses. 



Series IV. The only Palaeozoic rocks exposed in the township are of 

 Potsdam sandstone. The magnificent exhibition of this rock in the Ausable 

 chasm has been mentioned by many observers, and has been carefully 

 measured by Mr. Walcott.* Except for this river section, all the Potsdam 

 in the eastern part of the township is deeply buried by sand. The rock 

 exposed at Keeseville and in the chasm is quite homogeneous, of white, grey 

 or yellow brown color, and the fossils indicate that it is the upper portion 

 of the formation that is here shown. This, together with the lack of red beds 

 and conglomerates as the gabbro is approached, gives rise to the suspicion of 

 a fault contact. It is about on a line, also, with a fault which runs through 

 Peru and Plattsburgh, close to the lake shore. 



A very interesting exposure of Potsdam on the west side of the supposed 

 fault is worthy of description. The locality is not quite a mile west of 

 Keeseville, not far west of the race track, and a few hundred yards southeast 

 of School No. 5. Nestling in an indentation in the eastern face of the gabbro, 

 is a small mass of peculiar, very coarse conglomerate, capped by very red, 

 thin-bedded layers of feldspathic sandstone of the ordinary basal type. 

 The conglomerate carries very numerous, well-rounded quartz pebbles of a 

 reddish lilac tint, ranging in size up to two inches in diameter. With 

 these are occasional smaller fragments of orthoclase and a few dark colored 

 pebbles of decomposed rock, apparently of diabase. In streaks magnetite is 

 present in very large quantity, in well rounded grains. The coarsely granular 

 matrix looks black when fresh, but on weathering becomes mottled with 

 blotches of green chloritic material, which gives the predominant color to the 

 rock. In thin section, grains of quartz, microperthitic orthoclase, magnetite, 

 titanite and microcline, named in order of abundance, are seen to constitute 

 the matrix, and are set in a green chloritic-like cement, whose exact nature 

 is not clear. The coarseness of the conglomerate is astonishing when we 

 remember that it is composed entirely of gneissic debris, yet is in contact with 

 anorthosite, the nearest exposures of gneiss being one mile and a half away. 



In the northern and western parts of the township considerable Potsdam 

 is exposed. It is, for the most part, a hard, flinty sandstone of buff color, 



' Bulletin 81, United States Geological Survey, pp. 343-344. The boss of granite mentioned by Mr. Walcott. on the river 

 bank above Keeseville, iB of gabbro. 



