310 The Atncrican Geologist. November, 1899 
now show no remnant of an igneous texture or structure. The de- 
tailed structure of the region cannot be here given, but two very 
noticeable features are the quaquaversal dips surrounding the ovals of 
Archean rocks and the synclinal structure between these ovals. 
Iron ore, mostly hematite, is known from the ^Mansfield and 
Groveland formations of the Lower Huronian and from near the base 
of the Upper Huronian. The origin of the ore and its concentra- 
tion at certain points are regarded as in general identical with the 
origin and concentration of iron ores described by Van Hise from 
other parts of the Lake Superior region, notably from the Penokee 
and Marquette districts.* It is, however of interest lo note that in 
this paper Smyth presents facts to show that the iron ore of the 
Groveland formation in the Felch Mountain range is derived from 
original glauconite. While he admits that he does not consider this 
origin as demonstrated here, still he states that in his opinion the iron 
and silica of the rocks just mentioned were originally present largely 
in the form of glauconite. This agrees with the work of Spurr, who. 
a few years ago showed that the iron ores of the Mesabi range had 
this origin. t 
The Hemlock volcanic formation, which immediately succeeded the 
Randville dolomite in places and which is regarded at certain points 
as the time equivalent of both the INIansfield and the Groveland for- 
mations, has a varied and interesting lithology and is described "by 
Clements. While in the main of volcanic origin there are some inter- 
bedded sedimentary rocks, and, in fact, all gradations from pyroclastics 
to normal sediments occur. This author makes a special point of 
distinguishing between the eolian volcanic deposits, to which he re- 
stricts the term tuf¥, and the water laid volcanic deposits, which he 
terms volcanic conglom.erates, etc. The igneous rocks of the Hem- 
lock formation are both acid and basic, the former being rhyolite-por- 
phyries and aporhyolite-porphyries and the latter basalts — non-por- 
phyritic, porphyritic and variolitic. Both acid and basic pyroclastics 
occur. In some of the rhyolite-porphyries the quartz phenocrysts 
are surrounded by quartz aureoles and the groundmass is frequently 
micropoikilitic, this author regarding the latter texture as in this case 
an original one. Many of the basic lavas show an ellipsoidal parting 
as a result of which surface exposures of the rock resemble a con- 
glomerate with rounded boulders from a few inches to eight feet in 
diameter. Between these boulders or ellipsoids is a sparse schistose 
matrix of nearly the same color as the included masses. Such ellip- 
soidal greenstones are known from other parts of the Lake Superior 
district, and these here described are very similar to the forms of a 
volcanic agglomerate described by N. H. Winchell from the Vermilion 
iron range.J 
*U. S. Geol. Survey, Monographs XIX. and XXVIII. 
tGeol. and Nat. Hist. Survey of Minnesota. Bull. lo, 1894. 
JAmer. Geol., vol. 9, pp. 359-368, 1892. Geol. and Nat. Hist. Sur- 
vey of Minnesota, Final Rept. vol. 4, p. 255, 1899. 
