MAGNETIC SURVEYING ON THE COPPER-
BEARING ROCKS OF WISCONSIN.

Tel, IR, ANILIDIRICIBI=

INTRODUCTION.

THE writer and assistants devoted the field season of 1922 to
surveying with magnetic instruments an area of about 180 square
miles in northern Wisconsin. The country rocks here are the
Middle Keweenawan formations which in Michigan are being
worked for copper. Glacial deposits are heavy and consequently
exposures of ledge are few and small although probably the situ-
ation is not far different from that on Keweenaw Point. On
account of this condition the geology is generalized from scanty
data. Even the boundaries between Middle and Upper Keween-
awan divisions and between Middle Keweenawan and the older
rocks are not accurately known. And the drawing of internal
boundaries between the individual members of the Middle
Keweenawan series has been previously attempted only in limited
areas where exposures give some idea as to their location.

The main purpose of the work was to see if the magnetic
methods which have been used by this Survey during the past ten
years primarily in the search for iron formations would be of
assistance in locating formation boundaries and in determining the
structure of the copper-bearing basic lava flows.

For two reasons further knowledge of these Keweenawan for-
mations is desirable; the first is economic. Copper has long been
known to exist here and it is advisable to know if the geological
conditions may indicate possibility of commercial ore. The
second reason is that these formations contain the record of the
development of the Lake Superior basin structure and that record

* Presented before the Society of Economic Geologists, Ann Arbor Meeting, De-

cember, 1922, as a part of the symposium on Michigan Copper Deposits.
*Introduced by W. O. Hotchkiss.

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