66 



NEW YORK STATE MUSEUM 



calities is thus emphasized by this discontinuity of the subforma- 

 tion. It is expected that these beds will be found to thicken east- 

 wardly and to take on a more open-water character, whereas here 

 they have many marks of a shoreward, shoal-water deposit. A 

 small Maclurea-like gastropod is the only fossil observed in them. 



The beds that succeed, apparently unconformably, are of normal 

 " upper Beekmantown " character, mostly drab or gray dolomites, 

 sometimes with a pinkish cast, though there are one or more sharply 

 defined beds of white sandstone of " Potsdam " type in the series, 

 and a limited amount of sand in scattered worn grains is likely to 

 be found at any level, but its presence is not conspicuous. The 

 rock generally has a velvety surface on fresh fracture. These beds 

 are well seen along the Rutland Railroad at the Madrid-Potsdam 

 turnpike crossing and near Norwood, and constitute the highest 

 layers exposed on the Canton quadrangle. 



Paleozoic outliers. Returning to the southern boundary, it 

 is observed that the rock sequence above discussed lacks a 

 base, and that no exposures of its lowest beds, where the sub- 

 stantial Potsdam sandstone is to be expected, are known on this 

 quadrangle, and only the topmost part of the succeeding Theresa 

 formation. A glance at the State geologic map will show that the 

 Precambric crystalline rocks protrude far to the north on the 

 meridian of Canton. A reasonable assumption, to which the field 

 evidence offers no dissent, is that the Potsdam sandstone is entirely, 

 or nearly, cut out across this quadrangle by an elevated area or 

 monadnock in the Precambric erosion surface (" sub-Potsdam 

 peneplain ") north of Canton. There is abundant evidence about 

 Canton of the ruggedness of this sub-Potsdam surface, and this 

 would be merely repeating over a larger district what happens in a 

 smaller way here and there about Theresa and on Wellesley 

 island. 1 What comes the nearest to being an outcrop of the main 

 body of white sandstone below the Theresa is an extensive exposure 

 of cross-bedded saccharoidal sandstone with occasional large white 

 quartz cobbles, in the bed of the Grasse river just above the county 

 house. This small area is nearly surrounded by the crystallines, 

 the actual contacts covered, however, and thus must fit a deep 

 embayment in these, if it is not actually an isolated outlier. No 

 great thickness is visible, though the ledges form two separate 

 series of rapids. 



i N. Y. State Mus. Bui. 145, p. 60. 



