342 THE CAMBRIAN. [bull. 81. 



The typical sections are those of Potsdam, in St. Lawrence County; 

 Chateaugay Chasm, in Franklin County; Au Sable Chasm, in Essex 

 County, and Greenfield, in Saratoga County, in the State of New York, 

 and Heminingford, in Canada. 



Section at Potsdam. — At the type locality of the terrane at Potsdam, 

 in St. Lawrence County, there are but 60 or 70 feet of the rock exposed. 

 It occurs in layers from an inch or two in thickness up to 2 feet, and 

 the general color of the mass is yellowish brown to reddish brown, par- 

 ticularly the latter color in the thicker layers. 



Section at Chateaugay Chasm. — At Chateaugay Chasm, Franklin 

 County, one of the best sections of the formation upon the northern 

 slopes of the Adirondacks is exposed. I studied it in 1888 and found 

 that 5 miles south of the Ogdensburg and Lake Champlain Railway 

 the pre-Cambrian rocks are to be seen on the hills west of the river and 

 in a few small outcrops about one-half a mile to the north. It is a 

 bedded, reddish gneissoid and hornblendic rock, probably of Algoukian 

 age. Nearly one mile south of these outcrops the Potsdam sandstone 

 appears in the bed of the river, with a northwest dip of about 3°. The 

 exposures increase rapidly in thickuess, and bluffs from 50 to 75 feet 

 in height, formed of evenly-bedded layers, rise above the water. The 

 river has worn a canon through these beds for three-quarters of a mile 

 down to a mill and pond, where the cliffs break away, and the strala, 

 losing their dip, continue to the south, in nearly horizontal beds, to the 

 crossing of the railroad embankment. 



The fall in the river bed is rapid, aud at the High Falls of the Cha- 

 teaugay Chasm a bluff of 125 feet rises from the foot of the fall. The 

 lower 35 feet of the section is formed of coarse reddish and gray sand- 

 stone in massive layers. At the Narrows, three fourths of a mile far- 

 ther down the stream, the lower beds are 75 feet in thickness and pass 

 into massive bedded grayish and buff-colored sandstones that breakup 

 into thinner layers on exposure to the action of the weather. Many of 

 the massive beds are made up of irregular cross-bedded layers : this is 

 best seen about a mile below the falls. The strata below assume a south- 

 westerly dip of 2° to 3°, which carries the entire sandstone formation be- 

 neath a hard light gray sandstone a little north of the mouth of the chasm. 

 One hundred and fifty feet of sandstone, not included in the section 

 above the railroad embankment, is exposed in the chasm and 25 

 feet of the upper beds occur below the mouth of the chasm. This gives 

 a total thickness of 250 feet of sandstone as the known thickness of 

 the Chateaugay section. To this there is to be added an unknown 

 amount at the base and 25 feet of passage beds between the upper sand- 

 stone and Calciferous sandrock, to be seen just above the entrance of 

 Marble River into the Chateaugay. 



