GEOLOGICAL FORMATIONS. 671 



sotensis, the former being quite small, and usually having the shell 

 remaining; sometimes, however, the shells have been ground up so 

 that only a few fragments are found, disseminated through the shales. 

 The trilobites are seldom or never found entire, but usually the 

 cephalic portion, the pygidium, or still smaller fragments. The vari- 

 ous sandstone beds of the Potsdam are usually unfossiliferous. The 

 upper bed of sandstone seldom contains anything but Scolithus, which 

 in some localities is very numerous. This fossil is also frequently 

 found in all the sandstone beds. 



LOWER MAGNESIAN LIMESTONE. 



Geographical Boundaries. South of the Wisconsin river and 

 north of the main watershed, it occupies a tract of land lying about 

 the heads of all the smaller tributary streams, although seldom found 

 so far back as their springs. Passing down any of the streams, such 

 as Otter creek or Mill creek, we gradually pass below its surface; and 

 its outcrops are seen gradually higher in the hills, until, on reaching 

 the Wisconsin river, it forms the cap of all the bluffs from Sauk 

 City to Boscobel, usually appearing in bold and rugged cliffs, lend- 

 ing a very picturesque effect to the scenery of the river. It forms 

 also the valley of the Wisconsin for about four miles above its mouth, 

 and the valley of the Mississippi as far south as Glen Haven, where 

 it passes beneath the surface. Passing south of the divide which 

 separates the waters which flow into the Wisconsin from the Pecato- 

 nica, it is found in the branches of the latter stream, as far north as 

 the north line of to\vn 5 in ranges 5 and 6 E. Proceeding westward, 

 it is not again encountered until the Platte and Grant rivers are 

 reached, where it is found as the bed-rock of those streams, and in 

 T 5, R. 3 W., it covers a large portion of the township. 



In Green county, there is but one exposure of the formation. It 

 is in the valley of the Sugar river, about three miles above Brodhead. 

 It commences a short distance north of the center of Sec. 15, T. 2, R. 

 9 E., where it has a width of about half a mile. In passing into town 

 three, it widens to nearly two miles. It then becomes narrower, at- 

 tains its greatest elevation above the river at Albany, and finally dis- 

 appears beneath the surface of the river about a mile and a half above 

 that village. 



Lithological Characteristics. After passing through the transi- 

 tion beds separating it from the Potsdam sandstone, it assumes all 

 the qualities characterizing dolomite. It is very hard, compact, and 

 close-grained, of a grayish-white color. Beds of flint or chert are 



