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Wisconsin. Chief Geologist / Geology of Wisconsin. Survey of 1873-1879 ...
Volume IV (1882)

Crystalline rocks of the Wisconsin Valley,   pp. [628]-714 PDF (28.1 MB)


Page [628]

CRYSTALLINE ROCKS OF THE WISCONSIN VALLEY.
ROCKS OF TIHE VICINITY OF GRAND RAPIDS.
SKETCH MAP I.
The crystalline rocks of the area represented upon Sketch Map I
make their principal exposures in the bed of the Wisconsin river from,
Point Bass almost continuously to the eastern line of the map.   In
the southern half of this map, everywhere away from  the river, they
are buried beneath a rather thin covering of the Potsdam  sandstone.
In the northern half of the map, the Potsdaiu sandstone presents itself
in detached and as yet ill-defined areas, which grow smaller and
rarer towards the north. In the intervening spaces, the crystalline
rocks appear everywhere to be near the surface, the small cuttings,
for instance, upon the line of the Wisconsin Valley Railroad, strip-
ping them continually.  Although so near the surface in these areas,
they nevertheless make but few and unsatisfactory exposures, being
either covered with marsh peat or weathered down into soil.
The general characters of the rock exposures of the Wisconsin
river above and below Grand Rapids haWe been given somewhat fully
in Vol. II.'
The descriptions there given represent these rocks as a series of
micaceous gneisses, trending from.N. E. by E., to E. by N., and often
decomposed into kaolin, with a -very high northerly or southerly dip;
interstratified with which, in subordinate quantity, are hornblende
schists, and intersecting which are masses and veins of granite and
granitell. Our subsequent microscopic study modifies this descrip-
tion onlv so far as to include intrusive masses of black olivine-diabase,
which, at the time of the original examination, were not separated
from  the macroscopically similar hornblende rocks.2  The intrusive
1 Geology of Central Wisconsin, pp. 466-477. See, also, for description of
sandstone
of this vicinity, pp. 529 to 530, e63 to 565.
2 The hornblende rock of Figs. 7 and 8, p. 473, Vol. II, is an olivine-diabase.
Both
the diabase and the red granite-like rocks of these figures are to be taken
as intrusive,
The rock mentioned at the bottom of p. 475 (884) is also olivine-diabase,
as is also that
of Bed 10, Grand Rapids section, p. 473.


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