182 HALL AND SARDESON — THE MAGNESIAN SERIES. 



gists that an investigation along this line was not entered upon. In their 

 typical development these sandstones are masses of clear rounded grains 

 of quartz unmixed with any other material. From this typical condi- 

 tion they merge into clearly defined shales which are both aluminous 

 and calcareous in composition. These shales in turn pass into the car- 

 bonates of typical constitution. 



In the typical sandstones the grains are almost wholly quartz. Where 

 the color is white the mass is nearly pure silica. Each individual grain 

 is worn smooth and well rounded, so that under magnification it is well 

 polished. There is considerable variation in size ; in places quite wide 

 extremes are offered in coarseness. At Redwing and Mankato, Minne- 

 sota, even a conglomeratic texture is reached. In nearly all localities 

 where these sandstones are strongly developed the lower beds are the 

 coarser. Nowhere, however, has there been seen what may be considered 

 a basal conglomerate such as underlies the Potsdam at every point in 

 Minnesota where the base of that formation is exposed.* It is possible 

 there might have been laid down such a conglomerate whose immediate 

 sources would lie in limestones, shales and friable sandstones of every 

 varying phase. The ready solubility of the limestones, together with 

 the easy degradation of imperfectly lithified shales, make it quite improb- 

 able that in this case there was the usual permanent conglomeratic floor 

 on which subsequent layers were deposited.f The Jordan sandstone has 

 miany layers of very coarse sand intermingled with those of very ordi- 

 nary texture. 



A cementing of the sand grains is locally seen. At Utica and Jordan, 

 in Minnesota, the Jordan sandstone has sufficient firmness to be used in 

 bridge-building and coarse foundation work. 



The cement at these places seems to be a carbonate infiltrated from 

 the overlying rock. It fills the interstices of hundreds of cubic yards ; 

 it indurates vertical or horizontal sheets of the sand, or it forms concre- 

 tions of various, even fantastical, shapes. Where the sandstone thus 

 consolidated is broken it displays in the broad reflecting surfaces, some- 

 times inches across, the effect of crystallizing forces. The calcium car- 

 bonate is deposited among the quartz grains in the crystallographic 

 form of calcite over large areas and exhibits the characteristic cleavage 

 of this mineral. J Fine illustrations of such cementation occur below 

 Stockton and at Lanesboro, Minnesota, and elsewhere in both Minnesota 

 and Wisconsin, Again, as at Ottawa, in the upper layers of the Jordan 

 the grains are cemented with silica. A hand specimen taken within a 



* Paleozoic Formations of Southeastern Minnesota, Hall and Sardeson : Bull. Geo!. Soe. Am , 

 vol. 3, 1892, p. 336. 

 tR. D. Irving: Seventh Ann. Rep. U.S. Geol. Survey, 1885-'86, p. 397. 

 J Paleozoic Formations of Southeastern Minnesota, already cited, p. 345. 



