340 MINERALS AND GEOLOGY 



The Potsdam beds consist of coarse conglomerates and fine-grained 

 siliceous sandstones the latter in many localities sufficiently pure 

 for glass-manufacture and for the hearths of furnaces. The formation 

 is largely displayed in Hemmingford Mountain, and over large portions 

 of Huntingdon, Chateauguay, and Beauharnois, from whence it crosses 

 the St. Lawrence, and spreads over a large part of Soulanges and 

 Vaudreuil ; and from "thence, passing across the western end of the 

 Island of Montreal and Isle Bizard, it wraps around a large outlying 

 mass of Laurentian gneiss (forming Mont Calvaire on the north 

 shore), and continues uninterruptedly along the edge of the Laurentide 

 district as far east as the River Chicot, where the continuity of the 

 strata is broken by a fault, and limestones of the Trenton formation 

 are let down against the Potsdam beds. East of this point, the 

 formation only appears at one or two places notably on the St. 

 Maurice, where it exhibits a slight thickness of nearly horizontal 

 beds of conglomerate and sandstone, resting upon gneiss. Throughout 

 its range, as far east as the Chicot, it is accompanied by sandy and 

 dolomitic limestones of the Calciferous formation, and these cover 

 large areas south of the St. Lawrence, and in the country around the 

 junction of the St. Lawrence and Ottawa. East of this formation, on 

 the south aide of the St. Lawrence, limestones of the Chazy and 

 Trenton series, and dark bituminous shales of the Utica formation, 

 with succeeding sandstones and arenaceous shales of the Hudson 

 River formation, largely prevail the latter, especially, east of Riche- 

 lieu River. These formations cross the St. Lawrence, and range in 

 regular sequence along the north shore between the Calciferous out- 

 crop and the river bank. The intervening Island of Montreal, Isle 

 Jesus, Isle Bizard, etc., consist essentially of Chazy, Trenton, and 

 Utica strata the Hudson River beds coming up farther east. The 

 Chazy limestones of Caughnawaga and St. Domenique on the south 

 shore, those of Ste. Gene"vieve on the Island of Montreal, of Isle 

 Bizard, and of St. Lin on the north shore, yield marbles (red-spotted 

 or uniformly red) of good quality. East of the River Chicot, which 

 enters the St. Lawrence on the north shore, near the upper or 

 western extremity of the expansion known as Lake St. Peter, the 

 comparatively narrow strip of country between the Laurentian 

 gneissoid rocks and the river margin is occupied almost entirely by 

 Trenton, Utica, and Hudson River strata one or two small ex- 



