LITHOLOGY OF WISCONSIN.


more common accessory in the hornblende-schists than in the mica-schists.
What has already been said of the groundmass of mica-schist applies equally
well to hornblende-schist. Augite (sahlite) is a very common accessory, and
may very frequently be seen passing over into hornblende. So far as my own
examinations have extended, all of the hornblende of these augitic varieties
is of
a secondary nature, having resulted from the alteration of an augitic constituent,
and I have no doubt but that the same will prove to be true of all other
of the
Wisconsin hornblende-schists. The common accessories in these hornblende-
schists, in addition to apatite and sahlite, already mentioned, are magnetite,
hematite, titanic iron, the characteristic gray alteration-product of titanic
iron,
titanite and biotite.
The hornblende-schists are very much more abundant in Wisconsin than the
mica-schists, and indeed than any other kind of schistose rock, excepting
gneiss.
As typical instances may be mentioned the hornblende-schists of the Menomi-
nee region; 1 those interstratified with gneiss in the vicinity of Grand
Rapids
and Stevens Point on the Wisconsin river; 2 and those of the upper Rib river,
in Marathon county.3
Actinolite-schist. In certain rather unusual forms of amphibolite-schist
the
amphibole is actinolite instead of hornblende. These forms embrace three
types. In one of these the long actinolite needles form most of the rock,
the groundmass in which they are imbedded being composed of quartz grains
along with some chlorite. In another the quartz sinks nearly or quite out
of
sight, and is mingled with a large proportion of magnetite, mostly in clusters
of
well crystallized grains, while in the third the actinolite is associated
with a
large proportion of garnet. The actinolite-schists have been observed chiefly
in
the Menominee and Penokee regions, where they are associated with some of
the lower magnetic ores.4
AUGITE-SCHIST.
Wichmann first showed the existence in Wisconsin of schistose rocks in which
augite takes the place of the hornblende of hornblende-schist and of the
mica
of mica-schist.5 I have since found that such rocks are quite common in the
Upper Wisconsin valley, where they occur grading into hornblende-schist and
interstratified with and grading into augite-gneiss. The groundmass of these
rocks is chiefly quartzose, and entirely similar to that of the hornblende-schists
and mica schists. The augite is always of the variety sahlite.
I have already indicated my belief that the hornblende-schists of Wisconsin
are but altered forms of augite-schist.  As typical instances of Wisconsin
augite-schists may be mentioned those of Trapp river in Lincoln county,6
and
those of the Upper Rib river in Marathon county.'
CHLORITE-SCHIST.
Chlorite schists are dark-greenish, greasy-lustered rocks, in which chlorite,
in
closely aggregated or interwoven folia, is the chief constituent. Quartz
is the
principal groundmass mineral.  It may sink nearly out of sight, or may pre-
Vol. M, pp. 640-645.
' Vol. IV, pp. 629, 637,.644.
' Vol. IV, pp. 694-696.
4 Vol. III, pp. 105, 119, 123, 151, 482,487, 526, 545, 587, 639,
6Vol. III, p. 645.
' Vol. IV, p. 669.
7 Vol. IV, pp. 694-696.