PALEOZOIC ROCKS OF THE CANTON QUADRANGLE 1/ 



The Basal Contact 



The interesting nature of the Potsdam-Precambrian contact has 

 already been suggested. Marking as it does the greatest hiatus and 

 unconformity in the entire known geological record, it would in any 

 case attract attention but the variety of forms which it assumes in 

 the outliers south of Canton is remarkable. Following the post- 

 Laurentian uplift, which completed the metamorphism of at least 

 such, parts as now remain of the Grenville and associated rocks, 

 our region was approximately base-leveled. But either the process 

 of peneplaination lacked completion or was followed by some reju- 

 venation, for the irregularity of the sub-Potsdam surface, already 

 described by Professor Cushing for the Theresa region^ and else- 

 where in northern New York, is equally well shown hereabouts. 

 As today, so then, the Precambrian crystallines standing at high 

 angles outcropped in long belts or strike ridges, the stronger gneisses 

 and quartzite upstanding, the softer marbles and schists mostly 

 excavated by shallow stream-valleys or wind-erosion hollows. 



The suggestion is strong that at least the closing phases of this 

 land cycle were arid, and probably cold. Rock destruction by 

 atmospheric and solar agencies probably went on unhindered, the 

 resulting residual soil becoming charged with the insoluble red 

 oxide of iron derived from the various iron-bearing minerals of 

 these older rocks, whose other stable products were chiefly quartz 

 sand and ctay. The removal of the latter product was quite likely 

 effected by wind work comparable to that now going on over the 

 Great Plains, and its final resting place may be in part the Georgian 

 (Lower Cambrian) red shales in Vermont. The sand remained, 

 in part at least, and furnished material for the overlying forma- 

 tions. There are points (as at locality 75; summit of the quartzite 

 knoll southeast of the main mass) where the sandstone may be 

 found fitting into undercut spaces beneath the old crystalline ledges, 

 hinting at wind erosion of the desert type. Similar cases are 

 described by Cushing. There are other places (as at locality 74) 

 where the basal Cambrian is a reconsolidated talus breccia banked 

 against the foot of a steep quartzite ledge and showing a com- 

 pletely residual, nonassorted character. There are even places 

 (localities 74, 75, 70, 79) where through some 10 to 20 feet of beds 

 a complete gradation exists between ordinary fresh Grenville rocks 



* N. Y. State Mus. Bui. 145, p. 54- See also footnote 3, page 20. 



