98 THE PENOKEE IRON BEARING SERIES. 



At the Aurora mine an iuterestiug observation was made on tbe "granite" 

 which forms a low hill range a short distance south of the mine, and its manner of 

 contact with the rocks lying just north. 



The low granite range, wliich has been mapped as Laurentian by the Wi.sconsiu 

 geologists, rises about ~>0 feet above the mine and lies south from the mine about (JOO 

 feet. The section of strata intervening between the mine and the granite range is 

 made up about as follows, in scmthward (and descending) order: 



1. Iron ore, soft hematite, 100 to 150 Iw't. 



2. Sandstone (sometimes a quartzite), about 15 feet seen. 



3. Gray and greenish slates and quartzites, in beds from half an inch ti> 4 and 

 fi inches; distinc^tly sedimentary, oSO feet. 



■1. Gray quartzite like that of No. 3. 



5. Granite, hornblendic and massive. 



The sandstone (No. 1) forms the south wall of the Aurora mine. The ((uartzites 

 and slates (No. 2) are not all exposed at the mine, but at the Colby mine a section of 

 2r(0 feet, in a connected exposure, can be seen along the spur track west of the mine. 

 About 100 feet of similar strata are visible at the Aurora nunc. It is partly assumed, 

 therefore, that tlic whole interval of 580 feet consists of the same rock as seen at 

 the northern and southern limits. It seems to be a part of the Animike slates and 

 quartzites (pp. S.'j-Sfi). 



The liornblendegraiiiti' contains l)owlder foruis of dift'erent rock from the mass 

 of tlie granite, some of them licing of some dark-colored greenstone-like rock and 

 others of some earlier granitic rock. The great mass of the granite is maiidy ht)nut 

 geneous, and these bowlder forms appear most distinctly on the weathered surface of 

 the blutf. When these bowlder masses were not originally of greenstone tliey are 

 apparent by a blotched aspect which tlie granite presents, the blotches being cause<l 

 by some patches of roundt^d (mtline, much liner grained than the re.st, or by a marked 

 dirterence in tlie relative' amounts of feldspar and (juartz eomjjared to the Siime min- 

 erals in the most of tlie granite. The bowlder forms are, when distinguishable, from 

 two or three inches in diameter to twelve inches. Their longer axes lie luevailingly 

 in the same direction, but sometimes they have lost their first shape and have been 

 drawn out into points in one or more directions ov have been a little distorted by 

 unequal pressure.' 



This gi-anite also has a uniff>rm rift or grain, brouglit out by the constant or 

 prevalent elongation of the hornblende crystals in the direction about east and west. 

 The bowlders liave their longer axes in the same position. It appears, therefore, tliat 

 originally this granite was some conglomerate, which has been phistic or llnid-like. 



At the most southerly of the three low bluffs of this granite, each of which faces 



' Similar changes from conglomerates to gueiss aro mentioned by Dr. E. Hitchcock in Vermont, 

 in Am. Jour. .Sci., '2d sir., vol. x.\xi, p. 372. 



