J.P. Kimball on Iron Ores of Marquette, Michigan. 299 
ridges, as well as upon their summits, and are invariably revealed 
rthe removal from the surface of a rock of the alluvium and 
glacial drift by which it has been effectually protected. It is only 
in the outcrop of fissile schists that these appearances are want- 
ing, The'dense vegetation and the undisturbed covering of 
drift, aided by the deep snows of winter, almost entirely prevent 
the weathering of the rocks, so that the phenomena of glacial 
ation are exhibited in this region in uncommon perfection, 
ts of rock occurring near by in situ, and boulders of 
specular iron are scattered throughout the drift, which here gen- 
tually in fact appears to be made up of material derived from 
the immediate region, as seen especially in the circumstance that 
tisexceedingly ferruginous, while the source of the iron as a 
ocal detritus is evidenced by its exclusive presence as anhydrous 
—&squioxyd. Foreign boulders of gneiss occur however, consid- 
table in size and number within limited areas of distribution. 
__ The position of the beds of specular iron ore has already been 
_ tmer lie below drainage, in the bottom of the valleys, and in 
‘lhe case of the latter are commonly obliterated through the ero- 
_fonof elevated outcrops. Hence their outliers have been chiefly 
_‘Weserved along the flanks of the ridges where they have been 
_lneleast exposed to the agencies of denudation, protected as they 
“ete against a drift current coursing not with the valleys but 
: ; iquely across them, by the elevated outcrops of their under- 
2 ay 8 from the summits of which however they have been 
he denudation has been most extensive upon the 
_- Mr specular schists and the earthy red hematites, while the 
: “y €Xceptions to these conditions of erosion and preservation 
: already given, occur with specular schists which acquire from 
sae a a refractory structure, or from their mode of 
. fas, € property of resisting the effects of a sweeping de- 
m 
“aulon, 
; a bosses of specular-iron—the iron-knobs or mountains, as 
/. ate called—are the most striking examples of exception to 
: ot effects of denudation already noticed. They are in- 
8 Of the preservation of the anticlinal crest, and owe their 
Gener” © the fact that they are made up, not of pure and 
tated wt schists, but of specular iron ore closely interlami- 
“Indi with quartz or jasper—a structure capable of resisting de- 
: in ee” to a far greater degree than the homogeneous 
_*S The laminze of jasper alternating with pure specular 
