34 PARALLELISM OP THE QUEBEC GROUP. 



have placed the Quebec group below the Potsdam. As be has 

 published and circulated a letter addressed to Barrande, contain- 

 ing the above remarks, he is bound, in honor, to publish Barrande's 

 answer, and circulate that also. If he do not, then we must believe 

 that Barrande does not coincide with him. 



From Pillipsburgh, a range of exposures of limestone, runs 

 southerly to St Albans Bay in Vermont, a distance of about twenty 

 miles. Some of these small tracts of limestone are Trenton, while 

 others may belong to the Quebec group. At St. Albans Bay 

 there is a cliff or ridge, of reddish magnesian limestone and sand- 

 stone, running north and south, nearly parallel to, and a short 

 distance from the shore. This I believe to be the Potsdam. A 

 road, from the town of St. Albans down to the Bay, crosses this 

 ridge at a right angle, passing through a wide irregular ravine. 

 North of the road there is an exposure of greyish or whitish 

 limestone at the foot of the cliff, which may belong to the 

 Quebec group. It seems to plunge under the sandstone, as it is. 

 exposed within ten feet x>f it and dips towards it. In this 

 limestone I found a Pleurotomaria very like one that occurs in the 

 Quebec group. This place, might be appealed to as affording 

 proof, that the Potsdam overlies the Quebec group physically. 

 But I think there is a dislocation here. The Potsdam dips east- 

 erly at a gentle angle, but the limestone is greatly disturbed, and 

 is in some places vertical. Again, following the base of the cliff 

 southerly across the road we soon come to another exposure of 

 limestone of a different age. It is dark grey, blue and black 

 and often traversed by seams of white calc-spar. The strati- 

 fication is much confused, thus indicating the promixity of a fault. 

 Some of the beds hold Stenopora fibrosa, Strophomena alternata 

 and Asaphus platycephalus. These are either Trenton or Black 

 River. In one place the weathered surfaces exhibit sections of 

 Pleurotomiria and Maclurea and may be Chazy. It seems to be 

 underlaid by a black slate. The rocks of this exposure also seem 

 to plunge under the Potsdam, as do those north of the road. 



The Potsdam at St. Albans Bay thus comes in contact with 

 two different formations, within a distance of half a mile. Ac- 

 cording to Mr. Marcou's section there ought to be here 2000 feet of 

 slate between the limestone and Potsdam. I think that all the 

 facts, both palseontological and physical, that can be observed here, 

 indicate the existence of a great fault, with an upthrow on the 

 eastern side. 



