174 PALAEOZOIC TIME — LOWER SILURIAN. 



and also in Franklin co., N.Y., the conglomerate attains in places a thickness 

 of 300 feet. 



On the south shores of Lake Superior, east of Grand Island, the Potsdam 

 epoch is represented by the famous " Pictured" rocks, which form bluffs of 50 

 to 200 feet on the south shores and are variegated in color with vertical bands 

 and blotches. The color is often a red blotched with light gray, the red being 

 due to oxyd of iron, and the gray to a removal of the previous red color by 

 organic matter ; other colors are brown, greenish, and yellowish. Some of the 

 rock is very soft and fragile, and other parts are hard and gritty. The 

 " Pillared" rocks near the west end of Lake Superior are of the same age. On 

 the St. Croix, in Wisconsin, the rock is for the most part a laminated sandstone 

 of various colors, and either soft or hard. Some portions are partly calca- 

 reous ; and towards the top of the series true limestone layers are intercalated. 

 Green-sand like that of the Chalk formation occurs in some of the beds on the 

 Upper Mississippi. (Hall.) 



The Potsdam beds of Texas occur in Burnet co., Texas, where they consist of 

 sandstones covered by limestone. (B. F. Shumard.) 



Beds of sandstone and conglomerate, according to Dr. Hayden, skirt the 

 Black Hills of Dakota (lat. 43°-45° N., long. 103°-104° W.), overlying the 

 Azoic and containing characteristic fossils. Dr. Hayden has also observed a 

 similar sandstone and conglomerate in the Laramie Range of mountains, along 

 the margins of the Big Horn Range (lat 43°, long. 107°), and along the Wind 

 River Mountains. Those of the Big Horn Range afforded the usual Primordial 

 fossils. 



The Potsdam formation is 60 to 70 feet thick in St. Lawrence co., N.Y., 

 diminishing in some places to 20 or 30 feet; in Warren and Essex cos., 100 feet; 

 in the St. Lawrence valley, 300 to 600 feet; about 250 feet in Lake Superior; 

 700 feet, according to Owen, on the St. Croix, Wisconsin; 50 to 80 feet in the 

 Black Hills, Dakota; 200 feet at the Big Horn Range; 500 feet in Burnet 

 co., Texas. 



On Lakes Superior and Huron, in the copper region, there is a great thicken- 

 ing of the Primordial strata, in connection with eruptions of trap. The rocks 

 rise in some places to a height of 3000 or 4000 feet, and consist of these igneous 

 rocks mingled with the sandstone and a scoria conglomerate. These beds are 

 mostly of the Calciferous epoch. (J. D. Whitney.) 



b. Region of the Appalachians. — Along the Appalachian chain the great thick- 

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

 At Highgate, near the northern boundary of Vermont, the rock is a red sand- 

 stone containing fossils. This sandstone extends north into Canada, occurring 

 near Herrick's Mills, in the township of St. Armand. It also occurs west of 

 Swanton, in Vermont, where it is interstratified with black shales which contain 

 the peculiar fossils of the epoch, and some species are identical, according to 

 Billings, with those found on the north side of the Sfraits of Belle Isle. At 

 Snake and Buck Mountains, in Addison co., Vt., there are 700 feet of black 

 shale overlaid by a thick bed of sandstone and magnesian limestone, all of the 

 Potsdam epoch (Billings). At Georgia, Vt., there are black shales which have 

 afforded some large trilobites. South of these regions, east of the Hudson 

 River, along by the Taconic Mountains (2", 2" on map, p. 170), in the western 



