28 NEW YORK STATE MUSEUM 



Just east of Middleville the upper layer of the Beekmantown is 

 a coarse conglomerate, consisting of pebbles of quartz and chert 

 in a matrix of quartz sand grains, cemented by calcite. This con- 

 glomerate was noted at but the one locality, but elsewhere a very 

 sandy, sometimes slightly pebbly, layer, often full of pyrite (mar- 

 casite) is seen at this horizon, instead of the usual chert layer. 



The best exposures of the Beekmantown are those at Little 

 Falls, where, on the south side of the river, nearly every foot of the 

 450 feet thickness of the formation may be seen. The exposures 

 about Middleville are also very good, though the thickness has 

 dwindled to about 200 feet, and a section so nearly continuous can 

 not be seen. The contact on the pre-Cambrian is shown in the West 

 Shore railroad cut at Little Falls, and also in the banks of Spruce 

 creek at Diamond hill [see pl.3]. It formerly showed, in the 

 Boyer quarry at Little Falls, and even yet almost does so. All 

 along the line of contact on both sides of the river at Little Falls 

 the contact just escapes showing. 



The Spruce creek locality has been noted by both Vanuxem and 

 Darton.i The pre-Cambrian rocks there are garnetiferous Gren- 

 ville gneisses with quartz veins, with some basic layers, and some 

 rusty weathering beds full of pyrite. One of the latter is at the 

 contact for the slight length of its exposure, and plate 3 illus- 

 trates its rapidity of weathering as compared with the overlying 

 Beekmantown. The more resistant quartz veins project into the 

 overlying Beekmantown layer, so that specimens of almost solid 

 quartz can be broken out from it. Otherwise it, and the next two 

 layers, have an arkose character, and the lower of the two is quite 

 pebbly. They are also full of pyrite locally. While their ma- 

 terials are somewhat waterworn they consist merely of more or 

 less weathered fragments of the underlying rocks, the resulting 

 rock being mainly a product of weathering rather than of sedi- 

 mentation. But these beds grade rapidh^ into the ordinary 

 Beekmantown above, are themselves somewhat calcareous, and 

 make a perfectly logical basal layer for the formation. In plate 



^Geol. N. Y. 3d Dist. p.255. 



N. Y. State Geol. 13tb An. Rep't 1893. p.417. 



