30 POTSDAM GROUP. 



characteristic, and near St. Genevieve the rock is completely honey-combed with it 

 to the depth of three feet. 



57. It is largely distributed in Northern Michigan, and striking into Wiscon- 

 sin north of Green Bay gradually widens southerly as a surface rock, until it 

 reaches the central part of the State, where it has a width of 100 miles. It then 

 curves northwardly and enters Minnesota, forming the high hills on the Mississippi 

 River. It is unconformable with the rocks below, and rests upon an exceedingly 

 irregular surface, sometimes filling depressions in the quartzite or metamorphic rocks 

 of several hundred feet. Its upper surface is uniform, and graduates into the 

 Calciferous Group or the Lower Magnesian limestone, as the rocks in these States 

 are called. The exposed area in Wisconsin is about 12,000 square miles, the thickness 

 very irregular by reason of the great depressions and elevations at the base, and the 

 maximum thickness is fully 1,000 feet. The rock is chiefly composed of cemented 

 grains of silicious sand, but presents several varieties, as the calcareous, argillaceous, 

 ferruginous, and green sand, and the waters issuing from it in places contain a 

 small percentage of lime salts. In the argillaceous class the clayey material be- 

 comes so abundant as to render the rock shaly, and so impervious to water that 

 valuable springs occur at its upper exposed surface. In the calcareous class the 

 lime becomes so great in some layers that they are more properly limestones than 

 sandstones, and so associated with magnesia that they become arenaceous dolomites. 

 In the ferruginous class, at one extreme, the amount of iron oxide is barely suffi- 

 cient to color or cement the mass, and at the other so great as to make an iron ore. 

 In the green sand there are two classes, one in which the grains are colored by iron, 

 and the other consisting of deep green grains of glauconite. The green sand is not 

 restricted to the Potsdam in Wisconsin, for it also occurs in the Calciferous and St. 

 Peter's Sandstone. It is almost identical with the Cretaceous green sand of New 

 Jersey, and similar deposits in existing seas. The surface area in Michigan, Wis- 

 consin, Iowa, and Minnesota is estimated at 25,000 square miles, which is about half 

 the surface area on the continent ; but it is generally believed to exist under many 

 of the more recent deposits, and, therefore, to cover several hundred thousand square 

 miles. Springs and streams of soft water are abundant where it forms the surface 

 rock, and a good supply of soft water has been found wherever it has been penetrated 

 with the drill ; its existence, therefore, becomes a question of much economical interest 

 where a supply of good water is desired from artesian boring. The drill has never 

 reached it in Ohio, though a supply of good water is imperatively demanded in 

 some parts of the State ; and it is to be hoped an effort will be made to determine 

 whether it exists below the Calciferous, which has been reached with the drill 

 many times. 



58. It is exposed at numerous places in the Appalachian System from New York 

 to Tennessee. In New Jersey it reaches a thickness of 3,000 feet, and if both the 

 Chilhowee sandstone and Knox Group in Tennessee belong to it, it has a thickness 

 of 9,000 feet, but probably 5,000 feet of this belongs to the Taconic. In the south- 

 ern and south-eastern counties of Missouri it has a thickness of 700 feet. It appears 

 in several counties in Northern Texas, along the margins of the Big Horn, Laramie, 

 and Wind River ranges, at the Black Hills, and in other regions of the Rocky 

 Mountain System from Mexico to British America. The erosion by water and 

 weathering has left picturesque scenery in the sandstone at many places. The 



