i>98 



NEW YORK STATE MUSEUM 



matter of miieii doubt, did not the nature of the exposure make 

 it clear. The accompanying plates give a very imperfect idea of 

 the phenomena. 



In all the above cases of a certain or probable difference of age 

 in the gneiss formation, it is highly probable, from the nature 

 of the rocks, that they have come from a common source, and 

 that no great interval elapsed between the periods of eruption. 



About two miles south of South Hammond, near the road to 

 Rossie, the granite-gneiss shows a peculiar pegmatitic phase, 

 probably relatively late, packed with schist inclusions of 

 every conceivable shape and size, which are themselves penetrated 

 by dikes of granite and veins of quartz in great variety. For 

 an exhibition of all these phenomena, the locality can hardly be 

 surpassed, and is thus important as a basis of comparison. 



Nearly everywhere in the region studied, the Potsdam sand- 

 stone is a conspicuous formation, on account of its quantity and 

 its mode of occurrence. As a whole, it is a rather fine grained 

 sandstone, consisting almost entirely of well rounded quartz 

 grains cemented by secondary quartz. It is very uniform over 

 wide areas, showing almost no variation except in color, the pre- 

 vailing light gray or white being frequently replaced by red, 

 sometimes with complex striping and mottling. In general, the 

 bedding is practically horizontal, although gentle dips are not 

 uncommon. Occasionally steep dips are shown; a conspicuous 

 example being in the village of Theresa, where some rather sharp 

 folds appear. 



The upper part of the Potsdam is calcareous, and it doubtless 

 passes gradually into the Calciferous, as it does, according to 

 Cushing 1 in Clinton county. 



The actual base of the Potsdam, in contact with the underlying 

 crystalline rocks, was seen at only three points. One of these is 

 on the road from Redwood to Rossie, where it descends the Pots- 

 dam escarpment, just west of Mill-site lake. The flat Potsdam 

 rests on the eroded edges of a much decomposed ferruginous 

 schist. The bottom layer of the Potsdam, varying from 1 inch to 



^th an. rep't N. Y. state geologist. 1895. p. 511. 



