R. D. Irving—Huronian Series of Northern Wisconsin. 397 
ro 
ports this layer is evidently no less prominent and persistent 
than No. III of our series. It appears evident that the two are 
directly equivalent. 
ove the siliceous slates in the Penokee system we find a 
great belt (IV) of magnetitic schists, of the several varieties 
a 
to Nos. VI to XI of Major Brooks’ Marquette series. 
Immediately above the magnetic belt (IV) of the Penokee 
system, or separated from it by a band of black slate (V), 
which is not known to be sufficiently persistent to deserve 
whose underlying rock is a matter of conjecture (VII to 
XVIII). The black slates and interstratified quartzites are 
known at points along a belt over twenty miles in length. 
The equivalents of these members, in the Marquette region, 
appear to lie from XIV to XVIII of Major Brooks’s scheme, 
where we have quartzite and true clay slate, besides brown to 
black carbonaceous slates, often distinctly micaceous. 
Forming the uppermost members of the Penokee system, 
we find a great development of mica (biotite) schists, — 
1n thickness all of the lower members of the series, and including 
lark gray, aphanitic kinds, quite coarse, gneiss-like kinds, be- 
sides highly quartzose kinds. It is certainly a striking fact, 
that in the Marquette region the uppermost member (XIX), a 
mica schist, is the thickest of the whole series, covering often 
a width, according to the Michigan maps, of over a mile. 
