294 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



Northwestern Georgia, which, according to him, may be di- 

 vided into four parts, referred respectively to the Lower 

 Potsdam, 13,000 feet; Upper Potsdam, 2000 ; Quebec group, 

 12,000; and Cincinnati group, 15,000; the great mass of the 

 Chazy and Trenton limestones being unrecognized. These 

 figures, regarded by him as approximative, give a volume 

 of 42,000 feet. The rocks are described as chiefly gneisses 

 and hornblendic and micaceous schists, varying somewhat in 

 mincralosrical characters, but more or less auriferous through- 

 out, with the exception of the lower division. According to 

 Hunt, who has lately examined a section across the region, 

 there is nothing resembling the so-called altered Quebec 

 group of Logan (Huronian), nor yet the Laurentian ; the 

 whole series being apparently Montalban, with the excep- 

 tion of some portions of Lower Taconic (Taconian) ; and 

 the section similar to that described in the Record for 1877 

 (p. 173) in the Blue Ridge in North Carolina, to the east of 

 Roan Mountain. These rocks in Northern Georgia are, as 

 elsewhere southward in the Atlantic belt, deeply decomposed, 

 and the hornblendic gneisses, still retaining their cohesion, 

 are reduced in specific gravity from about 3.1 to 1.2, and 

 even to l,by the process of decay. There seems no reason 

 whatever for supposing this great series of crystalline rocks 

 to have been generated by the alteration of the 8000 feet of 

 sandstones, shales, and limestones which, at the western base 

 of the Blue Ridge, represent the Cambrian series of Sedg- 

 wick, from the base of the Potsdam to the top of the Cincin- 

 nati group inclusive. The evidence of the pre-Cambrian age 

 of these crystalline rocks is as undoubted as that of the sim- 

 ilar strata in the northwest {Record for 1877, p. 173). 



EASTERN CANADA. 



Selwyn has recently announced that late investigations by 

 the geological survey in Eastern Canada show that the crys- 

 talline rocks of the Green Mountain belt in that region, 

 which were by Logan maintained to be derived by altera- 

 tion from the fossiliferous Cambrian strata of the so-called 

 Quebec group, are really pre-Cambrian, and unconformably 

 overlaid by the latter, thus confirming the view of Hunt, 

 who had long maintained these crystalline rocks to be of 

 Huronian age. 



