LOWER SILURIAN. 167 



Minnesota, and some parts of the Kocky Mountains, as well as in Lab- 

 rador and Newfoundland ; also along parts of the Green Mountain 

 region, and portions of the eastern Appalachians in New Jersey, Penn- 

 sylvania, and farther south. As the Archaean rocks made the dry 

 rocky land of the era, these beds made of sand consolidated, were in 

 part at least the beach formations of the period. The bluffs of the 

 Au Sable chasm, west of Keeseville, are of Potsdam sandstone. In 

 northern Vermont, and beyond in Canada, there are slates of this era ; 

 and a reddish limestone, called the Winooski limestone, reaches south 

 to Burlington, Vermont, and affords a handsome red marble. 



1. Acadian Epoch. — The rocks are exposed to view in a number of valleys in 

 southern New Brunswick, and especially at St. John, where they were first proved 

 to be Primordial by G. F. Matthew. The name Acadian was given to the period by 

 Dawson. The rocks here are gray and black shales, with some sandstones, and have a 

 thickness, allowing for a fold, of 2,000 feet. There are fossiliferous rocks of this era 

 also in southeastern Newfoundland. In the same region, underlying beds, that have 

 been pronounced Huronian, have afforded two fossils (see Billings, Amer. Jour. Sci., 

 III. iii. 223). At Braintree, Mass., not far from Boston, the rock is siliceous slate and 

 clay slate. In both regions, the beds are much upturned. The lowest beds of the Wis- 

 consin Primordial may belong to this division, as stated beyond. 



2. Potsdam or Georgia Division. 



a. Eastern-Border Region. — On the Labrador side, and parts of the Newfoundland, 

 of the Straits of Belle Isle, there are strata of limestone, sandstones, and shales of this 

 era. They stretch across the north peninsula of Newfoundland to Canada Bay, where 

 the thickness, according to Murray, is 5,600 feet. 



b. New York, Vermont, and Canada. — The rocks occur adjoining the Archaean of 

 New York and Canada. They are here mainly hard sandstones, often gritty, some- 

 times pebbly (especially the lower beds), and only occasionally friable. The sand- 

 stone is generally laminated, and sometimes thinly so ; and of gray, drab, yellowish, 

 brownish and red colors. Much of it is a good building stone, as at Potsdam, Malone, 

 Keeseville, etc. North of the Archaean, in the northwest part of Clinton County, in 

 part of St. Lawrence County, near De Kalb, and also in Franklin County, N. Y., the 

 conglomerate is in places 300 feet thick. The rock bears evidence of being mainly of 

 shallow-water or beach origin. 



In St. Lawrence County, N. Y., there are, according to Brooks, conformable beneath 

 the Potsdam sandstone, strata of sandstone, metamorphic schist, limestone, and hema- 

 tite iron ore (the Caledonian or Parish ore-bed included), having in all a thickness of at 

 least 200 feet. See Amer. Jour. Sci., III. iv. 22. 



The formation is represented in western Vermont by the "Red Sandrock " ; the 

 Winooski limestone extending from Addison, through Burlington to St. Albans; also, 

 apparently over the latter, the black and gray shales or slates of Georgia, St. Albans, 

 and Swanton (referred by Emmons to the Taconic), which continue into Missisquoi 

 County, in Canada, lying to the west of other slates of the Quebec period. There are 

 Primordial black shales in Bald Mountain, Greenwich, Washington County, N. Y., de- 

 scribed as Taconic by Emmons; and shales and sandstone, with beds of limestone and 

 limestone-conglomerate, near Troy, N. Y., recently made known by S. W. Ford (Am. 

 Jour. Sci., III. ii. to vi.). The Troy beds, and also the Winooski marble and Georgia 

 slates, are believed to be inferior to the Potsdam sandstone. The quartzyte of the Green 

 Mountains of Vermont and farther south is supposed to be Potsdam sandstone. 



c. Region of the Appalachians. — Along the Appalachian chain, the great thickness of 

 the accumulations, and especially of the slates, is the striking peculiarity. 



In New Jersey, the rocks supposed to be Potsdam are sandstone, either soft or hard; 



