396 BR. D. Irving—Huronian Series of Northern Wisconsin. 
which the Penokee rocks were deposited; there are, indeed, no 
facts yet on record going to show that the two rock-systems 
In the Marquette region Major Brooks has made out with 
great care and skill a succession of beds which he numbers from 
V to XIX. at essentially the same succession exists in the 
Penokee region, there can be little doubt. An attempt to make 
out a scheme of equivalency for basins always disconnected, 
based on lithology alone, would, beyond doubt, be but time 
wasted. In the present case, however, the two districts are 
really continuous, and many of the layers in the two regions so 
very constant in their characters, that there can be no valid 
objection to the attempt. It is undoubtedly true that it is very 
easy to make many mistakes in such a scheme, owing to the 
dying out of certain layers, and the variations along the line of 
strike of some member of the series, which, if not originally 
present, may have been produced by the partial process of met- 
amorphism to which the whole Huronian system has been sub- 
jected. No scheme of equivalency can then be regarded as of 
any value, that is not based upon those few grand features of 
the stratigraphy, which are shown to be quite constant. The 
prominent facts in this connection, that have pressed them- 
selves upon me while studying over my field-results in connec- 
tion with the reports and typical collection of the Michigan 
survey, are here given. Major Brooks, whose wide experience 
in the several Huronian regions of the northwest, and whose 
skill in Huronian stratigraphy, are well known is, I believe, 
preparing a complete presentation of the whole subject of the 
equivalency of the strata of the various districts. The follow- 
ing are offered as suggestions toward the fuller treatment of 
the subject. 
n Wisconsin we find at the base of the series a great bed 
(ILD of light-colored quartzose slates and schists, over 400 feet in 
thickness, with characters so pronounced that it has been tra 
uninterruptedly for over fifty miles. The rocks of this layer 
vary from schistose vitreous quartzites, to argillitic mica-schists, 
while subordinate to it are the two lower meinbers, the white 
arenaceous quartzite (II) and tremolitic crystalline limestone 
(i1]). Now in the Marquette region the base of the series is a 
*Ge ‘Survey of M: vol. 83 of Report on Iron Rocks, and p. 
i ot bepnaun One ichigan, i, p. 183 of Report 8, 
ae ae eee 
