CALCIFEROUS GROUP. 33 



which and east of the St. Croix it forms the surface of nearly two large counties of 

 Wisconsin. It follows the Mississippi north of Minneapolis for several miles before 

 it is covered with later formations. The conspicuous perpendicular walls of rock, 

 cropping out from the hills and bluffs along the Mississippi from the St. Croix to the 

 mouth of the Wisconsin, belong to this Group. Throughout the exposures in Wis- 

 consin, Iowa, and Minnesota, it is conformable with the underlying Potsdam, and un- 

 conformable with the overlying rocks. The lower surface is plane, while the upper 

 surface is undulating, and in some instances the undulations are said to swell in short 

 distances into elliptical domes, rising 100 feet above their bases, like billows on the 

 sea. These undulations are the work of denudation during the interval that 

 elapsed before the deposition of superimposed strata. The Group in Wisconsin is 

 frequently called the Lower Magnesian limestone, and some one in Minnesota has 

 called it the Shakopee Group, because the stone has been quarried at a village bear- 

 ing that Indian name. The Magnesian limestone is usually sufficiently pure to burn 

 to a serviceable quicklime. The chief impurities are quartz, clay, iron, and green 

 sand. The dolomite occurs in the earthy, granular, crystalline, and crypto-crys- 

 talline forms, and chert is irregularly distributed. Argillaceous material is not 

 abundant, except in shaly bands, where it may constitute 20 per cent of the whole ; 

 and the amount of silica disseminated through the rock varies from 1 to 10 per 

 cent. The difference in the composition and hardness of the layers causes the sur- 

 face rocks to present great irregularities, which are much enhanced and exaggerated 

 by weathering, and hence outliers have a rough and often grotesque exterior. 



§ 64. The Group is displayed in grand proportions in the southern counties of 

 Missouri, where it consists of an upper and lower division of magnesian limestone 

 with an intermediate division of sandstone. These received the names, in descending 

 order, of the "Second Magnesian limestone," the "Second Sandstone," and the 

 "Third Magnesian limestone." The upper division is generally composed of beds of 

 earthy magnesian limestone, interstratified with shale-beds and layers of white chert, 

 with occasionally thin beds of white sandstone, and near the lower part thick, 

 cellular, silico-magnesian limestone-beds. It constitutes many of the bluffs of the 

 Osage and its tributaries, and also of the Missouri from Osage to Jefferson City. 

 It is often a lead-bearing rock, as in Cole County. The thickness rarely exceeds 200 

 feet, though on the Meramec it is 300 feet. The middle division is usually a 

 brownish sandstone, stratified in firm, regular beds from 2 inches to 3 feet in thick- 

 ness, though sometimes friable. The surfaces are often ripple-marked. The thick- 

 ness rarely exceeds 150 feet. The upper part often occurs in thin strata with beds 

 of intercalated chert abounding in fossils. The third division is generally a thick- 

 bedded, coarsely crystalline, bluish-gray magnesian limestone, with occasional thick 

 chert-beds. It is the chief lead-bearing rock of South-east and Southern Missouri, 

 and is frequently exposed along the streams in bold escarpments from 200 to 300 

 feet high. The ores of lead, zinc, copper, nickel, and cobalt, occur in fissures and 

 caves, or disseminated in small masses in the limestone itself. The lead occurs some- 

 times in masses of galena accompanied with copper pyrites disseminated through 

 layers of limestone, while the ores of nickel and cobalt occur in clay slate. At 

 other places bands of red clay inclose calamine (silicate of zinc), galena, and 

 heavy spar (sulphate of baryta). The maximum thickness is about 600 feet, though 

 it seldom exceeds 300 feet. The maximum thickness of the three divisions is more 



