168 • PALEOZOIC TIME. 



or a red crumbling shale, as in the Green Pond Mountain Range; or a tirm conglom- 

 erate. Near Flanders, a kind crumbles easily to sand. 



In Pennsylvania, there are, in the Primal series of Rogers, 2,000 feet of lower slates, 

 overlaid by 90 feet of sandstone, and this by 200 to 1,000 feet of upper slates (H. D. 

 Rogers). In Virginia, there are 1,200 feet of lower slate, 300 of sandstone, and 700 of 

 upper slates (W. B. Rogers). 



In East Tennessee, J. M. Safford has described, as of this age, the "Chilhowee" 

 sandstones and shales, several thousand feet in thickness (consisting of sandy shales, 

 sandstones, and light gray quartzyte), resting on the Ocoee conglomerates, sandstones, 

 and micaceous, talcose, and chloritic slates. 



d. Interior Continental basin. — The sandstone rocks in New York and Canada, 

 above mentioned, properly lie in the northeastern border of this basin. 



In Wisconsin (as first announced by D. D. Owen), a broad band of the Potsdam 

 sandstone borders the east, south, and west sides of the Archaean, south of Lake 

 Superior, crosses the Mississippi, about the Falls of St. Croix, into Minnesota, and then 

 stretches northward and southward, passing in the latter direction toward Iowa. The 

 rock over the interior of Wisconsin and Minnesota is, for the most part, a very 

 crumbling and imperfectly coherent mass of sand. It includes much green sand in 

 its lower part, similar in general character to the green sand of the Cretaceous form- 

 ation (Hall). It forms bluffs on the Mississippi, in Iowa, below the Upper Iowa 

 River. This loose condition of one of the most ancient of rocks, in Wisconsin and 

 Minnesota, shows how ineffectual are ordinary waters, even through the lapse of ages, 

 in causing solidilication. The sands are often whollj' siliceous, with only 1 or 2 per 

 cent, of impurity, and, when crumbling, make a good material for glass. 



Hall (Regents' Rep. 1863) makes out three divisions of the Wisconsin beds: 1, the 

 lower, containing species of Conocoryphe and no Bicellocephali ; 2, the middle, charac- 

 terized by species of Conocoryphe, Dicellocepihalus, Agraulos, Ptychaspis, Aynostus, 

 with the earliest Graptolites ; and 3, an upper, clearly separated from the great central 

 mass, and containing species of Dicellocephalus, Triarthrella, Aglaspis, Lingula, Serpu- 

 lites, Euomphalus. 



The Pictured Rocks, forming bluffs 50 to 200 feet high, on the south shore of Lake 

 Superior, in Michigan, and the Pillared Rocks, at the west end of the lake, have been 

 considered as of the Potsdam era, but are now referred to the next period. 



The Potsdam beds of Texas occur in Burnet County, 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 Archaean, and 

 containing characteristic fossils. 



e. Summit and western slopes of the Rocky Mountains. — Primordial rocks occur in 

 the Big Horn Mts., at the head of Powder River, long. 107°; as quartzytes (probably 

 of this age), near long. 112° W., along the Wahsatch, Teton, Madison, and Gallatin 

 ranges, resting unconformably upon the upturned Archaean gneisses and granytes; also 

 in Nevada, long. 116° W., as announced by J. D. Whitney. 



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

 Warren and Essex Counties, 100 feet; in the St. Lawrence valley, 300 to 600 feet, or 

 more; about 250 feet on Lake Superior; 700 feet, according to Owen, on the St. Croix, 

 Wisconsin; 50 to 80 feet in the Black Hills, Dakota; 500 feet in Burnet County, Texas. 



Markings in the rocks. — In the Acadian rocks, near St. John, 

 N. B., the coarser layers are frequently covered with ripple-marks 

 and shrinkage cracks, and also with scratches that appear to be the 

 tracks of some water-animal ; and, besides, there are worm-burrows. 

 (SeeScolitfacs, p. 177.) The facts, as G. F. Matthew states, are evi- 

 dence that the beds are of seashore origin. The shales of Georgia, 



