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ARCIATAN (EOZOIC) ERA.
CHAPTER IV.
LAURENTIAN AGE.
FIG. 1.
GENERAL SECTION OF THE FORMATIONS OF WISCONSIN. The inclination of the unflexed
beds is
exaggerated. L. Laurentian. H. Huronian. P. Potsdam sandstone. L. Mg. Lower
Magnesian
limestone. St. P. St. Peters sandstone. T. Trenton limestone. G. Galena limestone.
H. R.
Hudson River (Cincinnati) shales. N. Niagara limestone. Hr. Lower Helderberg
limestone. Hm.
Hamilton limestone (cement rock).
Synoptical Notes on Laurentian Formation. Name derived from Laurentide
Hills of Canada. Rocks of metamorphic class, mainly gneisses. Thickness un-
determined, but great. Strata much folded and contorted. Occupies a large
area in Northern Wisconsin.
For details see Vol. II, pp. 248-9 (Chamberlin), 461-501 (Irving); Vol. III,
pp. 5-6,
92-99 (Irving), 224 (Julien), 248-50 and 300 (Wright), 399-400 (Strong),
434 and 661
(Brooks); Vol. IV, p. 109 (Wooster), 585-601 (King), 617-715 (Irving).
Iocation of Early WIsconsin -land. That part of the earliest
known formation which falls under our special study formed an
island, or group of closely associated islands, lying       immediately
south of the present Lake Superior, and occupied portions of North-
ern Wisconsin and the upper peninsula of Michigan.'
General Character of the Rocks.       We have already referred      to
this as the granitic foundation upon which the rock structure of our
State is builded. The rocks, as we now find them, consist of a series
of granites (quafel-mJ), gneisses (foliatedlfel~-mi-qua), syenites (fel-
qua-horn), hornblendic, micaceous and chloritic schists, and allied
rocks. With these are associated igneous diabases (plagi-auq), and
similar rocks, together with diorites (plag-1-horn) of undetermined
origin. Among these rocks, the gneissoid granites vastly predomi-
nate, so that the whole series in a general view is conveniently
termed granitic.
I It is not improbable that this was at one stage united with the adjacent
main-
land on the west and north.