424 THE GEOLOGY OF MINNESOTA. 



[Potsdam and St. Croix formations. 



At 1810 feet, fine sandstone, with grains partly light gray, and partly dusky brown, the 

 latter averaging slightly larger than the former. In the pulverized drillings these differently 

 colored portions remain separate, though abundantly shaken; giving the powdered stone a mottled 

 and streaked appearance. 



At 1827 feet, medium grained, reddish, friable sandstone. 



At 1860 feet, fine-grained, reddish gray, soft sandstone. 



At 1872 feet, ochery and siliceous, very fine-grained, dull red shale. 



At 2000 feet, red shale, with occasional grains of sand; resembling pipestone in color and 

 fineness; but with scarcely more hardness than common clay. 



At 2150 feet, similar red shale, slightly arenaceous. 



At 2200 and at 2204 feet, was the same red shale, containing fine grains of white quartz. 



From the depth of 915 feet in this well, to its bottom at 2204 feet, its 

 section thus consists of sandstone and shale, mostly reddish in color, and 

 not remarkably indurated. At 915 feet and again at 1700 and 1720 feet are 

 beds of red shale; but from 1010 to 1650 feet, and from 1810 to 1860 feet, 

 the specimens are siliceous, principally reddish and soft or friable, ordinary 

 sandstone, with water-worn grains. At and below the depth of 1872 feet, 

 the remaining 332 feet consist largely and perhaps wholly of dull red, 

 slightly arenaceous shale, which extends below the bottom of the well. 

 This formation of sandstone and shales, thus shown to have a thickness of 

 about 1300 feet, appears to be, stratigraphically and lithologically, the same 

 with the .nearly horizontal red sandstone, including frequent beds of shale, 

 which borders the south shore of lake Superior almost continuously from 

 Fond du Lac to Grand island and again rises into view at the falls of St. 

 Mary. 



St. Croix sandstone and shales. The white sandstone in this well at 495 

 feet, the shale at 560 and 600 feet, and the light-colored sandstone from 640 

 to 850 feet, belong to the St. Croix formation, which is exposed in the bluffs 

 of the St. Croix and Mississippi rivers. These beds, with the St. Lawrence, 

 Jordan, Shakopee and St. Peter formations, are the western equivalents of 

 the Calciferous, Quebec and Chazy rocks in the northeastern United States 

 and Canada. The presence of the Potsdam sandstone and shales beneath 

 the St. Croix in this and several other artesian wells in southeastern Min- 

 nesota, and the uniformity of the sections thus shown, demonstrate that 

 these are two distinct formations, and make it almost certain that the St. 

 Croix beds lie conformably upon the latest Potsdam deposits. 



St. Lawrence limestone. This formation is the lowest of the three mem- 

 bers of the Lower Magnesian series which are exposed in the valley of the 

 Minnesota river and its tributaries in Blue Earth county. It was encoun- 



