REPORT OF THE DIRECTOR AND STATE GEOLOGIST 



r99 



2 feet, is a conglomerate, with rather small quartz pebbles. 

 Above this the rock has its normal character. 



About five miles east of this point and one mile west of Grasse 

 lake there is another contact of a very similar character. The 

 third contact is on Wells island, about one mile east of Thousand 

 Island Park near the road leading to Westminster Park. Here 

 the Potsdam rests on decomposed granite-gneiss or schist and 

 the lower 5 feet of the former is sandstone. Above this 

 is quartz conglomerate, with pebbles ranging up to G inches 

 in diameter. Judging from these localities, it is evident 

 that the Potsdam of this vicinity is somewhat different 

 from that farther east where, as Gushing 1 has shown, the 

 lower part of the formation is always a coarse conglomerate 

 and usually red. The latter variety, excepting the red color, 

 is, however, shown at several points on Wells and Grind- 

 stone islands, though in no case has it been found in contact with 

 the pre-Cambrian rocks. These localities show 7 a very massive 

 conglomerate, with pebbles ranging up to a foot or more in 

 diameter, the pebbles usually being quite rough and angular. In 

 composition, these pebbles are singularly uniform, as they are 

 with rare exception fragments of quartzite, identical with that of 

 the neighboring ridges, or of vein quartz, such as occurs abund- 

 antly all through the pre-Cambrian rocks. Careful search re- 

 vealed a few small pebbles derived from the tourmalin contact 

 zones described above, indicating a degree of resistance for this 

 rock which is also shown by a general projection of the zones 

 above the surrounding rock surfaces. Not a pebble was found of 

 the granite-gneiss or schist, the conglomerate being thus es- 

 sentially different from a true basal conglomerate. So far as 

 composition is concerned, the conglomerate is practically identi- 

 cal with the sandstone; both are almost pure quartz, and differ 

 only in grain. 



From this very pronounced character of the formation it seems 

 justifiable to conclude that the rocks of the pre-Cambrian land 



Potsdam and pre-Cambrian boundary, 16th an. rep't X. Y. state geol- 

 ogist. 1S96. 



