Page:The American Cyclopædia (1879) Volume XVI.djvu/705

 WISCONSIN 681 greatest numbers are found near the sources of the Chippewa and St. Croix rivers, the whole surface being studded with them, so that in some districts it would be difficult to travel 5 m. without finding a lake. A kind of wild rice (zisania aquatica) grows in the shallow- waters, affording sustenance to innumerable water birds. The principal lakes are Winne- bago, the largest, St. Croix, Pepin, Poygan, Pewaukee, Geneva, Green, Koshkonong, and the Four Lakes. The geology of the state is simple, the series of rocks extending from the Laurentian to the Devonian. In the north central portion of the state there is a large area of archasan (azoic) rocks, having an ex- treme length E. and W. of nearly 240 m., and a breadth N. and S. of 160 m. These rocks are metamorphic in origin, and consist chiefly of granite, gneiss, syenite, diorite, and other hornblendic and allied rocks, quartzite, por-- phyrite, and a variety of schists and slates. The falls and rapids of the St. Croix, Chip- pewa, Black, Wisconsin, Wolf, Peshtigo, Me- nomonee, Montreal, and other rivers are due to the unequal hardness of these rocks. Be- sides the main area, there are in the south central part of the state outcrops of limited extent entirely surrounded by later formations, through which they project. The greater part of the archsean rocks of the state are regarded as belonging to the Laurentian period, though the Huronian are well developed. The uncon- formability of the two has been demonstrated, but their boundaries have not yet been satis- factorily traced at all points. To the latter period belongs the Penokee Iron range, which abounds in magnetic iron not yet worked. Similar iron deposits also occur on the Meno- monee river and at Black Eiver Falls. The copper-bearing series is found in Ashland, Bay- field, Douglas, Burnett, and Polk cos., and con- tains copper, but to what extent is not yet fully ascertained. Resting unconf ormably upon the archa3an rocks is the Potsdam sandstone, form- ing a belt on almost every side from 10 to 60 m. in breadth. The general form of the sand- stone district is that of a crescent, its horns on the Menomonee and St. Croix rivers, and its greatest breadth in the region of the Wiscon- sin river, near the middle of the state. The sand is generally pure, frequently suitable for the manufacture of glass. It often contains calcareous beds with fossil remains of a very ancient fauna ; the decay of these beds min- gling with the sand renders the soil fertile. This rock often forms bold cliffs and promi- nent peaks. The strata present ripple marks, cross laminations, and other evidences of de- position in shallow water. Next above the Potsdam sandstone is a heavy deposit of lime- stone, locally 'known as the lower magnesian limestone; it contains copper ores in a few places, and also lead. This is succeeded by the upper sandstone, having many of the charac- teristics of the Potsdam, upon which are the blue and Galena limestones (or dolomites) of the Trenton period, chiefly in the latter of which are found, in the S. W. part of the state and adjoining portions of Iowa and Illinois, the fissures containing deposits of lead, zinc, and copper ores. In this district, though yield- ing large quantities of lead, the soil is rich and productive. These mines were first discovered by Le Sueur in 1700, but attracted little atten- tion till 1826, from which time the quantity of lead produced increased rapidly till about 1845, since which it has declined. Upon the Galena limestone rest the blue and green shales and limestones of the Cincinnati group, and upon these at Iron Ridge, Hartford, and other points in eastern Wisconsin, rest the Clinton iron ore beds, which attain at the first named place the unusual thickness of 25 ft., consist- ing of regular horizontal beds, which are ex- tensively mined with the very greatest facility. Above this, and forming the surface rock over a large area in the E. part of the state, is the Niagara limestone, which is highly magnesian, and contains some of the purest stratified do- lomites known. It affords excellent building material and quicklime of the first quality. Near Milwaukee, covering a limited space, occurs a rock now known as the Milwaukee cement stone, from the hydraulic properties which it has recently been shown to possess in a high degree. It belongs to the Hamilton period of the Devonian age. The limestone district of Wisconsin includes nearly all those portions lying S. and E. of the Fox and lower Wisconsin rivers, with considerable tracts along the Mississippi and W. of Green bay. All these rocks are older than those of the coal formation ; hence no coal is found in this state. With the exception of the lead region, and the counties along the Mississippi river, the state is covered with a heavy deposit of clay, sand, gravel, and bowlders or "drift;" and it is gen- erally this deposit rather than the underlying rocks that gives character to the soil. Among the pebbles masses of native copper are often found, associated with silver, clearly showing that this drift had its origin at the north. The drift, in a modified form, furnishes the clay from which cream-colored bricks are made, of great beauty and durability. A geological sur- vey of the state is in progress ; a full report is now (1876) in preparation. Lead ore is the most important mineral product of the state, found chiefly in the counties of Grant, La- fayette, and Iowa ; it is mostly the sulphuret (galena), though the carbonate (called white mineral) often occurs. Iron ores are found in great quantities and of easy access at Iron Ridge in Dodge co., at Ironton in Sauk co., at the Black river falls in Jackson co., and in the Penokee Iron range, in Ashland co., near Lake Superior. Magnetic ores also occur in the archcean region in the vicinity of the Me- nomonee river, in the N. E. part of the state. The ore has been smelted only at Iron Ridge, Ironton, and Black River Falls. Native cop- per is found in limited quantities in the N.