PALEOZOIC ROCKS OF THE CANTON QUADRANGLE 43 



The sandstones of the Potsdam represent the residuum of insolu- 

 ble, perdurable quartz sand from prolonged deep and thorough 

 weathering of the Precambrian gneisses and quartzites. Their red 

 color marks the complete oxidation of the iron content of those 

 rocks freed by the total destruction of the amphiboles (hornblende 

 etc.), pyroxenes and dark micas of the gneisses. At favored points 

 this iron was concentrated in the regolith of the rotting crystallines, 

 and incorporated as ore deposits in the basement of the Potsdam 

 sands accumulating above. The sands themselves bear evidence 

 at many places of being wind-drifted, more particularly in their 

 lower portions. The cross-beddings in the Hannawa quarries are 

 quaquaversal, on a giant scale and at depositional angles too steep 

 for water-saturated sands. Quite confidently these are petrified 

 sand dunes. The total absence of fossils, the undercut erosion, 

 so evidently of a sandblast character, in the Grenville quartzites 

 at some of the contacts, and the fine, even nature of the sand itself 

 right up to the contacts, all suggest eolation. In short, our early 

 Potsdam land was a vast desert of wind-shifting sands when it 

 saAk slowly beneath the advancing Cambrian sea. Its surface was 

 not flat, but rather it had the form of a corroded peneplain above 

 which rose, often steeply, knobs and ridges of the more resistant 

 quartzites and granite-gneiss, sometimes to heights of 200 or 

 300 feet. These summits were probably bare rock, rounded or 

 etched by the wind and exfoliating under the glare of the sun. Quite 

 possibly they had been not long since glaciated by the Cambrian 

 glaciers. They bore little or no vegetation even of the low types 

 such as mosses and lichens then in existence, and rainfall upon 

 them would appear to have been scant and infrequent. 



The suggestion of glaciation, made less improbable by the dis- 

 covery of unquestioned tillite (boulder-clay rock with striated peb- 

 bles) in the Cambrian strata of Norway, Australia etc., is drawn 

 partly from the frequent association of glacial conditions with red- 

 rock accumulations, and partly from the tillitelike nature of the 

 curious deposit at Mr Dillabaugh's (locality 85). Here, in the pi'O- 

 tection of an almost overhanging encircling ledge of the gneiss, 

 is a mass that consists at base of a regolithic or talus-heap breccia 

 with residual-earth matrix and secondary quartz and hematite 

 crystallizations of great beauty; but above this lies an indurated 

 quartzose conglomerate of very perplexing nature. In lack of 

 stratification, complete jumbling of material of all sizes, and com- 

 pact character of the matrix this closely resembles a glacial ground- 

 moraine, but the convincing witness of striated pebbles has not 

 vet been secured. 



