472 GEOLOGY OF CENTRAL WISCONSIN. 



a dirty wliite, and the interior, though still firm and hard, of a somewhat earthy tex- 

 ture. In some fragments a few minute grains of unaltered felspar can be detected 

 with the magnifier. 



From the gneiss exposures shown at the water's edge in the sketch just given (Fig. 6), 

 the following measured section was taken entirely across the great ledges of gneiss 

 which form the Grand Rapids of the Wisconsin. This section runs N. 20° W., on the 

 hne A B, or at right angles to the general trend of the layers, wliich is usually quite 

 plainly to be seen. The exposures are not ontL-ely continuous along the line of section, 

 but are nearly so, and gaps could be filled with ledges a little distance on one side or the 

 other. At the time of our examination, the river was low, and an unusual amount of 

 rock laid bare. The measurements given indicate horizontal widths. Since the general 

 dip is southeastward the first bods of the section are the highest in the series. 



Feet 

 1. Gneiss: at the beginnmg of the measurement (833) coarse-grained, distinctly 

 laminated, black-white-and-puik-mottled; striking N. 80° E., (lipping 60° 

 S. E. The mica of this rock is greenish and brownish, and aggregated 

 into large blotches ; the felspar is both pink and white, the latter not plainly 

 striated, and occurring m large facets; the quartz is abundant and limpid. 

 Pyrite is present in small quantity. Twenty-five feet from the beginning, 

 the felspar is much increased in quantity (834). At fifty feet the grain of 

 the rock becomes much finer, and the mica is largely restricted to the sur- 

 faces of the quite distinct laminae (835). This variety gives place soon 

 again to a coarser kind (836), similar to that at the beginning of the section 

 (833), but with the quartz somewhat more prominent. At seventy feet the 

 bedding directions are quite plain and show a strike of N. 75° E, and a dip 

 of 60° S. E. Here the rock laminse sire contorted, and the mica very 

 abundant, almost excluding the felspar. A few puJrish granite veins occur, 

 from }4 inch to 2 inches wide, conforming to the bedding of the rock. The 

 vein matter (838) is predominatingly of pinkish, flaky quartz, felspar being 



subordinate, and the mica restricted to the sides of the vein, 100 



'J. Covered, by water, 20 



:-i. Gneiss : at the beginning of the measurement coarse-grained, scliistose, con- 

 torted in places,' jjinkish- white, very quartzose, carrying pyrite (839), strik- 

 ing N. 85° E. Twenty feet beyond, this changes to a finer-grained, dark- 

 grajdsh, schistose kind (840), embracing some thin veins of white quartz 

 and pinkish granite. At 30 feet, the strike is N. 80° E., the dip 65° S. E. 

 At 80 feet occurs a reddish granite vein six feet wide. The vein-matter 

 (843) shows a fine-grained texture and dark reddish color, and appears to be 

 a mixture of very fine, pinldsh felspar facets, and translucent quartz grains, 

 no mica being apparent. At 100 feet the grain becomes coarser again until 

 at 130 feet (844), the rock is again like that at the beginning of the section 

 (8.33). Beyond, the grain again becomes finer (845), the pinldsh felspar at 

 the same time increasing in amount, and occurring to some extent in a por- 

 phyritic manner, j^g 



4. Covered, by water 250 



5. Hornblende rock (846): very fine-graiaed, highly crystaUme, distinctly bed- 



ded; in places thin bedded; dark-colored or black; strike N. 60° E., dip 



60°S. E '. .. 20 



(i. Covered, on islands ^a 



7. Hornblende rock: similar to the last described, but much weathered and 



broken by joints; strike and dip obscure 20 



6. Covered 05 



