THE LOWER SILURIAN ROCKS. 605 



considerable extent on the side of the hill on sections 17 and 18, and beyond doubt is to 

 be found in the neighboring country at many other fovorably located points. This is a 

 matter of some importance, since in the present quanies the stripping has become a 

 heavy expense. 



At Madison, the Mendota is exposed in several small railroad cuttings (622) on the 

 north side of Sec. 22. On the S. W. qr. Sec. 23, on the point of a low ridge projecting into 

 the marsh, is a quarry of some size, showing the lower 10 feet of the Mendota limestone. 

 The upper layers, are thin, the lower heavier, and all very irregular. The rock (613) is 

 a dark yellow to brownish, rough- textured, concretionary limestone, containing many 

 red patches of iron oxide, which proceeds, apparently, from the oxidation of pyrite. The 

 composition is: silica, 4.18; alumina, 2.17; iron peroxide, 1.45; carbonate of lime, 55.68; 

 carbonate of magnesia, 35.52; water, 0.58=100.58. Greensand in scattering grains, and 

 light green earthy patches are seen throughout, whilst a regular greensancl layer (612) 

 at the bottom of the quarry marks the base of the formation. Towards the side of the 

 ridge the limestone layers have an inclined position due to undermining. This quarry 

 was one of the first points at which the Mendota was recognized, and may be regardeu 

 as typical of the lower part of the formation. The rock has many points in common 

 with the Lower Magnesian, being quite as free from sand as that rock. 



The Artesian-boring in the Capitol park at Madison has a depth of 1,015 feet, pene- 

 trating in its course Glacial Drift, the Potsdam sandstone, and 200 feet of the Archaean 

 rocks. The water in the well comes within some 60-70 feet of the surface, from where 

 it is pumped for use in the boilers at the Capitol and for drinking purposes. It is re- 

 garded as a "mineral " water, but is not one, being freer from solid ingredients than 

 ordinary well water, and containing nothing unusual except a small quantity of iron 

 bicarbonate, the iron of which, on exposure to the air, peroxidizes, and produces a 

 brownish sediment. A record of this boring has already been given in the annual re- 

 port of Dr. Lapham, page 50; the greenish mineral from the rock at the bottom of 

 the well is, however, probably not prehnite, as there given. The Artesian boring at the 

 Milwaukee and St. Paul depot begins at a level 74 feet below the top of the boring in 

 the Capitol park, and brings water to within 7 feet of the surface. The following is 

 an abbreviated register of this boring : 



Feet. 



Drift: sand and clay, with bowlders; the lower part nearly all loose sand, so that 

 it is difficult to tell where the drift ends and the underlying sandrock be- 

 gins ; about 70 



Potsdam sandstone: specimens from depths of 200, 250, 290, 350, 360, 380 and 

 390 feet show very fine, white quartz sand, stained here and there with 

 deep brown points of iron oxide, and entirely non -calcareous; some of the 

 sand is a little coarser, and all as s?en under the microscope is made up of 

 very much rolled grains, the larger ones of which are almost wholly spher- 

 ical. Specimens from 600 and 680 feet are also of limpid quartz, but the 

 grains are very much coarser and less rounded. The lowest layer of the 

 formation struck is soft red shale, like that found in the Capitol well. 



Thickness in all, about 665 



Arclwean : dark colored rock, like that in 'the Capitol well 50 



Depth of boring 785 



The lower layers of the Madison sandstone are quarried on the S. E. qr. Sec. 28, 

 Madison, on the south shore of Lake Wingra, and the same rock is finely exposed (614, 

 617) with a thickness of 23 feet, largely pure white sand, and overlaid by 17 feet of non- 

 arenaceous thin-bedded Lower Magnesian, in the railroad cutting, S. E. qr. Sec. 35. 

 About 35 feet below the bottom of this cut, 10 feet of Mendota 'is exposed on the 



