DESCRIPTIONS OF THE FORMATIONS. 15 



The general color of the stone at Potsdam is yellowish-brown, but the 

 tint of each layer differs somewhat from those adjacent to it, so that the rock, 

 upon the fractured edges, wears a slightly striped aspect. It is the finest quarry 

 stone in the state, being so perfectly workable and manageable. 360. It is an ex- 

 cellent building material, holding mortar well, and makes a dry house. 29. Under 

 the Potsdam, and upon the primary rock, is the position of the specular and red 

 oxide of iron. V. 267. 



In Minnesota, the lower portion of the formation is 400 feet thick, and is hard 

 and often vitreous, and usually of a brick-red color, with very distinct layers, often 

 separated into slaty layers by partings of red shale, strongly marked with 

 f ucoidal impressions, frequently ripple-marked and cracked. The upper part of 

 the formation, there called the St. Croix sandstone, is white or buff in color, often 

 friable, and constitutes a heavy bedded or massive sandstone of rounded quartzose 

 grains. N. H.Winchell. 



In Minnesota and Iowa, the Potsdam proper, omitting the St. Croix sandstone, 

 is a friable, crumbling mass, of no value for building purposes except as 

 sand, consisting of a pure silicious sand in minute grains, with a very slight 

 amount of cementing matter. Unless protected by some more resisting rock 

 above it the Potsdam appears in steep slopes, or low, gently swelling hills and 

 mound-like eminences. Those portions which are hard and enduring are cemented 

 by oxide of iron, and have a brown color. 



In Wisconsin, the Potsdam is 800 to 1000 feet thick, and has a much larger 

 surface-development than elsewhere, as will be seen by the great number of 

 railway-stations on it. It extends over 12,000 square miles, and contains many 

 fossils not found in New York. Where the Potsdam in Wisconsin is on the surface, 

 and not covered by drift, there is usually a loose, sandy soil, with a sparse growth 

 of small oak and pine timber. This formation is one that has. been very properly 

 allowed to retain its original name almost undisputed all over the United States, 

 except that Professor Owen at first called it the LOWER SANDSTONE, in the North 

 West to distinguish it from the 3 c., St. Peters or Upper Sandstone. 



In Michigan, the Potsdam is the red sandstone, which is emphatically the 

 chief rock that appears upon the immediate coast of the whole south shore of 

 Lake Superior, and forms the Pictured Rocks and the Falls of St. Marie. Here it 

 is of inconsiderable thickness, but it regularly thickens in going westward. 

 Houghton, 4th R., 500. Some have referred the Lake Superior sandstone to the 

 age of the Chazy, but the late studies of Rominger show that it is really of 

 Potsdam age. The Chicago Tribune office building is of this Lake Superior 

 sandstone, and the Court House at Milwaukee is another conspicuous specimen. 



In Pennsylvania, the Potsdam, is a compact, fine-grained, white and yellowish 

 vitreous sandstone, containing specks of Kaolin. 



The Potsdam formation is supposed by some to Jbe represented in the Green 

 Pond Mountain of New Jersey by a local deposit of coarse conglomerate, 3000 feet 

 thick, but others deny that this mountain is Potsdam. It is less than 30 feet thick 

 where it is seen rising from beneath the limestones of the Lehigh River, but 

 increases in thickness westward and southward, until it comes to be represented in 

 Tennessee by many thousand feet of alternate coarse and fine deposits. See 

 Safford's Geol. R. of Tenn. 



