26 NEW YORK STATE MUSEUM 



Cambrian ridges, while the lower beds are limited to the depressions. 

 The bulk of the rock is a hard, white, sugary-looking sandstone, 

 locally with a brown tint, composed of nearly pure, quartz sand, 

 thoroughly cemented by silica. The majority of the exposures are 

 of low, fiat, glaciated surfaces, and cliffs, or exposures showing any 

 considerable thickness of the formation, are ver}- exceptional. 

 Excellent exposures may be seen to the south of Brier Hill Station, 

 and these expand into a broad sandstone platform to the south on. 

 the Hammond sheet. There are excellent exposures also along the 

 west shore of Black lake, above the narrows. Good exposures 

 also occur at the northeast end of the Macomb granite mass, though 

 here the formation shows only the upper 20 feet. Here also are 

 the only • actual contacts with the underlying Precambrian which 

 were seen within the map limits. On the Ogdensburg quadrangle, 

 great, bared exposures occur 2 miles south of Heuvelton, and from 

 there run southwest for 3 miles, or rather for several miles, for 

 the same belt continues down into Macomb, giving way' to Pre- 

 cambrian just about at the southern edge of the map. 



Interbedded with the sandstone on the Alexandria quadrangle is 

 a peculiar conglomerate, whose pebbles are large sized cobbles 

 rather than pebbles, which may reach a foot in diameter and often 

 show a diameter of 3 inches. The pebbles are almost exclusively of 

 Grenville quartzite, and are usually subangular, though well-rounded 

 ones also occur. Th^y are set in a matrix of sand, and the whole is 

 bound together by a cement which is more often calcareous than 

 siliceous, though both sorts occur. Where the cement is calcareous, 

 as it is in the cliff along the St Lawrence below Chippewa bay, the 

 cobbles weather out rather rapidly ; where it is siliceous the rock is 

 very resistant. This conglomerate is usually not basal in the for- 

 mation, but has a var}dng thickness of ordinary sandstone under- 

 neath it. It is a most astonishing formation and difficult of 

 explanation. 



This river cliff, containing the conglomerate, continues on to the 

 Brier Hill sheet, in Hammond, as far as Oak Point, beyond which 

 it also appears intermittently. There is a fine exposure of it near 

 Point Comfort. 3 miles southwest of IMorristown, where a small 

 fault runs east from the river bank and brings up the conglomerate 

 on its north side, well above the river. It shows precisely the same 

 characters as in Alexandria. Not far beyond the Potsdam dips 

 beneath the bed of the river, after which the Theresa beds are at 

 the river level for several miles. Underneath the conglomerate at 



