162 



KEMP'S ORE DEPOSITS. 



contains much bituminous matter. The buff is locally called 

 " Quarry Rock " and is prolific in fossils. Under the Trenton lies 

 the St. Peter's sandstone, 150 feet below which is the Lower Mag- 

 nesian, 100 to 250 feet, and still lower the Potsdam, averaging 

 700 to 800 feet. The Potsdam rests on the quartzites and schists 

 of the Archaean. The ore beds especially favor the shallow, syn- 

 clinal depressions of the east and west folds. They occur in crev- 

 ices, the great majority of which run east and west. The pro- 

 ductive ground comes in spots which are separated by stretches. 



FIG. 41. Gash veins, fresh and disintegrated. The heavy black shading 

 indicates galena. After T. C. Chamberlain, Oeol. Wis. , Vol. IV., p. 454. 



of barren ground. The lead ores are chiefly produced by the 

 crevices in the Upper Galena. In the Lower Galena the zinc ores 

 become relatively more abundant, and they are also in the Tren- 

 ton. The ores do not extend in any appreciable amounts either 

 above or below these horizons. The upper deposits favor the ver- 

 tical gash vein form ; the lower tend rather to horizontal open- 

 ings, called flats, which at the ends dip down (pitches) and often 

 connect with a second sheet (flat) lying lower. There are several 

 minor varieties of those two main types of cavity, which mainly 

 depend for their differences on the grade of decomposition, which 

 the walls have undergone, and whether there was an original open- 



