Cushing — Potsdam Boundary. 



19 



two gneissic ridges and with gneiss within a quarter-mile on each side, is a 

 knoll of hard, well indurated, red and white banded sandstone which has been 

 somewhat quarried. The location of the exposure, the 25° dip and the fact 

 that the horizon in the Potsdam is somewhat above that of the previous ex- 

 posure, indicate that the presence of the sandstone here is owing to dislocation. 



The higher portion of the formation is well shown in the river at Malone 

 and thence northward, it being quarried considerably about a mile north of 

 the city. Beyond, occasional outcrops are found along the river for a distance 

 of several miles before the Calciferous is reached near Westville. For about 

 half of this distance the inter-stratification of cn-ev dolomite with the white 

 sandstone indicates the presence of the passage beds to the Calciferous, which 

 apparently occupy the centre of a shallow synclinal trough, as the north- 

 westerly dip beyond is replaced for some distance by one to the south-east. 

 The wide extent of surface underlaid by the Potsdam here is thus explained. 

 In the passage beds at this place are iron-grey sandy dolomites presenting a pecu- 

 liar appearance, and such are found to characterize this horizon throughout 

 northern Xew York. On the fresh fracture, glittering cleavage faces 

 are shown a half inch or more in length and dotted in a psuedo-peecilitic 

 fashion by numerous rounded quartz grains, giving a peculiar satiny lustre. 

 The thin section furnished the explanation. The rock is a fine mosaic of 

 dolomite crystals in which are streaks numerously set with somewhat 

 rounded grains of quartz. In the quartzose bands are frequent areas 

 in which the cement enclosing the grains has the same extinction throughout. 

 In these cases the matrix is found to be of calcite instead of dolomite. In the 

 sandy streaks then, rather coarsely crystalline secondary calcite has been 

 deposited around and including the quartz grains, its good cleavage manifesting 

 itself when the rock is broken. 



Bangor and Brandon. 



The only Pre-cambrian rocks exposed in Bangor are the gneisses of Cornish 

 hill which extend northward four miles beyond the average line of the bound- 

 ary. Following the west side of the ridge the boundary passes into northern 

 Brandon, then swerves to the westward and continues in that direction across 

 the township into Dickinson. 



The gneisses exposed are quite homogeneous and consist mainly of micro- 

 perthitic gneiss in the eastern, and microcline gneiss in the western half of the 

 township, the two having the same color and appearance and grading into one 

 another. Hornblende is present in variable amount in all the exposures, and 



