Correspondence. 385 
continuously between N'ermilion lake and Gunflint lake and the points 
of unconformity with the overlying Animike have been mentioned in 
several places in the reports of the Minnesota survey.' There is no 
geological fact in connection with the work of the Minnesota survey on 
the crystalline rocks which is more confidently affirmed, or attested 
by more recorded facts of observation than that of the continuance of 
the Keewatin from Vermilion lake to the north side of Gunflint lake, 
and the unconformity of the Animike with the Keewatin at all places 
where the two formations can be compared. That it is the Keewatin, 
and not the Animike that holds the ore at Tower, this is no place to 
inquire. The reports of the Minnesota survey attest that conclusion 
in many places. If the facts stated in the Minnesota reports be not 
sufficient "evidence of equivalence" of the formations at these two 
points to satisfy Prof. Van Hise, we shall have to abandon the effort 
to do so. The hypothesis of Prof. Irvdng that some portion of the 
Animike has got entangled with the older schists at a point (at Tower) 
twenty miles distant from its known line of strike south of the Giant's 
range, and that the Animike once was continuous over the Giant's 
range to that point, is, so far as we know, the only evidence (if it may 
be so classed) of the Animike age of the Tower rocks. Prof. Irving's 
mistake consisted in having supposed the differences in crystalline 
structure that he noticed between two sets of rock samples from the 
Vermilion lake region (viz. some from the true crystalline schists and 
some from the Keewatin, or semi-crystalline schists) were the ana- 
logues of the differences he noted between two other sets of rock sam- 
ples from the Marquette region (viz. some from the dioritic schists 
and greenstones and some from the iron-bearing fragmental Huronian) ; 
whereas these differences themselves are not only not of the same 
class, and are due to different causes, but the compared formations 
from which the samples were derived, do not occupy the same strati- 
graphic position. 
Prof. Van Hisc's reference to Prof. Irving's description of carbonate 
of iron in the Vermilion ore constitutes an important correction of our 
paper. We had not observed the fact reported by Irving. It ought 
to be taken into account in further research on this subject. We have 
referred to and read carefully the whole discussion of this carbonate in 
the Tower ore by Prof. Irving,* and it bears to us internal evidence 
that there was possibly some confusion in the labeling of the speci- 
mens from Minnesota, and that the carbonate and the "actinolitic 
magnetite slates" containing so much carbonaceous matter were not 
from the vicinity of Tower, but from the Animike at some points on 
the Mesabi iron range. We have never found such in the region of 
Tower, nor at any place in the Keewatin formation, but they are a 
^Compare the 16th report Minnesota survey, pp. G7-71, 79, 80, 235, 
255-259, 323, 357-359; seventeenth report, pp. 87, 88, 91, 104, 108, 
110, 186. 
* Am. Joxir. Sci. vol. xxxii, 1886, p. 269. 
