24 NEW YORK STATE MUSEUM 



From 20fo to 25fc of the feldspar is oligoclase and the remainder 

 is anorthoclase. The composition is that of a rather acid quartz 

 syenite, and the rock is provisionally classed with the green 

 gneisses associated with the Grenville rocks. 



Paleozoic rocks 

 Potsdam sandstone. Though it has been sometimes held that 

 this formation is present in a small way in the district, the writer 

 found nothing that w^ould warrant its mapping as a lithologic 

 formation distinct from the Beekmantown. At the base the Beek- 

 mantown is often more sandy than usual and even pebbly, some- 

 times a thin shale band creeps in as at Little Falls, and sometimes 

 a thin, disintegration conglomerate or breccia band, composed 

 mainly of fragments of the underlying rock, is locally found. But 

 these phenomena are precisely what would be expected, as an old 

 land surface sank beneath sea level and began to receive deposit. 

 These lower layers vary greatly in character from place to place, 

 always are somewhat calcareous and usually are prevailingly so, 

 and seem to have nothing in common with the coarse, pure quartz 

 sands of the Potsdam formation. Furthermore, beds of this char- 

 acter are not confined to the base of the formation but equally 

 sandy, sometimes pebbly, beds are found here and there at various 

 horizons. Moreover, since the Beekmantown formation overlaps 

 on the pre-Cambrian, the formation thins going north from the 

 Mohawk, and hence successively higher and higher beds become 

 basal. The small area near Salisbury, which Darton has mapped 

 as Potsdam is, though basal, at an horizon at least 200 feet above 

 the bottom of the formation as shown at Little Falls. The same 

 characters run through several layers in the distinct overlap of 

 the formation at Diamond hill, the ordinary character being re- 

 sumed at a little distance from the spot, and the rock is still some- 

 Avhat calcareous and has not the lithologic character of the Pots- 

 dam.i Nor does the basal bed at Little Falls appear to represent 

 the real base of the formation, deep well records to the west seem- 

 ing to indicate an increased thickness in that direction, under 



^N. Y. State Geol. 14th An. Rep't 1894. Map at p.33. 



