PALEOZOIC ROCKS OF THE CANTON QUADRANGLE I3 



occurred while the sand was as yet unconsolidated, so that all the 

 layers have been stretched or dragged, along a zone of appreciable 

 width. In the glacial kame sands of Cobb's hill, Rochester, similar 

 faulting is demonstrably due to ice thrust; but it does not follow 

 that the same agency operated in these Paleozoic sands. 



A mile south, at Mr Martin's place, locality 8i, there crops up 

 a vitreous exceedingly tough cream-white sandstone or quartzite, 

 without visible bedding. Its relations to all other exposures are 

 unknown, but it is not to be confused with the more metamorphosed 

 Grenville quartzites. 



The above outcrops have been described in detail because they 

 afford many of the chief types of rocks included under the name 

 " Potsdam sandstones " and illustrate the rapidity of variation in 

 short distances or in successive layers. Together they constitute 

 an instructive and easily accessible group, presumably a single out- 

 lier, closely surrounded on all sides by the outcrops of the crystal- 

 lines and underlaid mostly by the weaker Grenville limestones. 



Beginning with the area just described and extending thence 

 southwestward in a direct line up the strike valley of the Grass 

 river and Harrison creek, following the marble belt, is a chain 

 of at least five other Potsdam remnants, four of them (72-75) 

 within the limits of the quadrangle and the fifth (71) just outside. 

 Doctor Martin reports another a mile farther up the valley. The 

 suggestion that these remnants sketch the course of a pre-Potsdam 

 valley is strengthened as one sees the old gneissic rocks rising, 

 sometimes a hundred feet higher, on either hand and the sand- 

 stone abutting against their slopes with interesting contact 

 phenomena. 



Each member of this chain is individual. At the nearest (locality 

 75), on the south bank of the Grass river a mile southwest of the 

 large area already described, there is a heavy ledge of white Pots- 

 dam with small dip to south running almost " end on " into a steeply 

 dipping mass of deep red basal Potsdam striking approximately 

 at right angles to the white, with merely a shallow gully intervening. 

 There are suggestions of a coarse conglomerate (see plate 3) at 

 the base of the white beds, quite different from the talus breccia 

 (plates 5 and 6) at the bottom of the red immediately adjoining. 

 But the question of an unconformity here between white and red 

 is still debatable. Just south of the white ledge lies a large rec- 

 tangular block that shows a few sinuous worm trails, the only 

 fossil seen in the " Potsdam " rocks of our area. 



