GEOLOGICAL EXPLORATIONS AND LITERATURE. 97 



coarsely granular, silicoons saiidstfure, tlic grains bi'iiig subaiigular aftor <lisinti'gration. 

 lu some i)la«'os it is tirm and (M)rn't'tly iK'ars the iianu- of (juartzik'. It is stained, 

 loeaUy, Willi miicli or little iron, and with manganese. The " silieifieation" process 

 seems here eertaiuly to have pioduced fragniental silica, and subsequently, beiug 

 interrupted, to have been followed by the process of ferruginization, which stained 

 this sandstone with iron and manganese, sometimes almost constituting it an iron ore. 

 It is not possible to say this ferruginization is a secondary, oi- rather a third, step, 

 later than the general ferruginiziug process, and that this quartzite lias acijuired the 

 ir(m by reason of the accidental contiguity since, for the tpiartzite graduates into the 

 rock of the mine, the acquired substances (iron and manganese) being the same as in 

 the real ore. The general circumstances of the situation will not allow of such a 

 separation. 



About 1350 feet farther north is an opening known as the North Golby. This is 

 a long deep i>it, where the iron has been worked out superticially, and timbered shafts 

 are beiug prepared for deener mining. 



On tlie spur track from the Colby to the Valley nunc is a short cut in siliceous 

 greenish and yellowish slate, now largely ferruginous. This is sometimes earthy, like 

 some seen at the (.lolby mine, but it is very siliceous \nth "chalcedonic" silica, some 

 of it being almost wholly white, although the prevalence of hydro-mica and perhaps 

 of other fine grained mineral i)articles intimately disseminated through the siliceous 

 parts gives a greenish color to even' the hardest and most ((uartzose parts of this lock. 

 This dips so as to conform to the rock in the Colby inine, and probably lies under the 

 foot wall of the South Colby. The full thickness of the bedding here involved, 

 making allowance for the oblique direction in which the road goes across it, is about 

 250 feet (pp. 54-55). 



At the Valley mine . . . in a ditch beside the railroad track at Bessemer, is a 

 sinill exposure of the bottom conglomerate of the Cupriferous. It contains numerous 

 pebbles of quartz and of fine porphyry like the copper-bearing conglomerate at the 

 Calumet and Ilecla nunc. The dip can not be made out, as the exposure is small and the 

 pebbles arc rather evenly distributed throughout the whole. This conglomerate nuist 

 lit' in unconformity on the Gogebic iron-bearing rocks, and if it be the equivalent of 

 the conglomerate at Negaunee (Cascade), as it appears to be, it indicates the greater 

 age of the llnronian quartzitc (the N(^w York Potsdam sandstone) than the Cu))riferous 

 formation, i. c, than the greater part of tlu^ Cupriferous, since the Cupriferous strikes 

 east and west near Bessemer, forming a conspicuovis range of hills of eruptive rock 

 but about a mile farther north, the dip being such as to cause it to overlie this con- 

 glomerate. 



The Aurora mine, at Ironwood, has a sandstone or a soft (piartzite, rather 

 coarse, identical with that on the south wall of the South Colby mine, for its south or 

 foot wall. 



MON XIX 7 



