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‘ Dr. C. T. Jackson on the Geological Structure, §c. 65 
Art. IX.—On the Geological Structure of Keweenaw Point; 
oe 
by Dr. C. T. Jackson, U. S. Geologist. 
(From the Proceedings of the American Association, 2nd meeting, held at Cambridge, 
1849, p. 288.) 
Tuts remarkable promontory extends from the south side of 
. Lake Superior nearly into the middle of the Lake, from 46° 40’ 
to 47° 2% north latitude, and is comprised between 87° 55’ and 
cal interest. On the immediate coast, excepting at a few points, 
the first rocks that meet the eye of the geologist are a coarse Con- 
glomerate, made up of large rounded and smobth pebbles of red 
porphyry quartz, altered slate and sandstone, masses of epidote 
rock, syenite and hard greenstone trap, fnostly of the porphyritic 
variety, and regular strata of fine grained red and grey or mottled 
sandstone, devoid of any fossil contents.’ ‘ 
The direction of the strata of sandstone and conglomerate is 
parallel to the line of uplift of the trap rocks, or E.N.E., W.S.W. 
Its dip is toward the W.N.W., at various angles, being greatest 
hear the trap rocks, which come between their strata and divide 
the great masses of sandstone throughout the whole length of the 
promontory. The strata remote from the trap on Keweenaw 
Bay, and the opposite side of the point, at the ‘portage, are hori- 
zontal or but slightly waving, while near the trap rocks the dip 
of the strata is generally as high as 30°, and sometimes more. 
€ conglomerate rock is limited to the borders of the trap range, 
and is of the same geological age as the finer grained sandstones, 
and alternates with them. __ : 
_ At the line of junction of the trap rocks and sandstones, the 
sandstone and trap are interfused, producing that singular an 
very important metamorphic rock amygdaloid, a rock closely re- 
sembling the vesicular lava of volcanoes, but having its cavities 
filled with a great variety of curious and interesting minerals. 
In the memoir published by Mr. Alger and myself on the min- 
eralogy and geology of Nova Scotia, in the American Journal of 
Science, vols. xiv, xv, 1828, will be found an account of the ori- 
gin of amygdaloidal rocks, like those of Keweenaw Point, and 
it May not be uninteresting to compare the trappean ranges on 
€ Superior with those of Nova Scotiaa On inspection of the 
map it will be seen that the great trappean band on Keweenaw 
Szconp Sznres, Vol. X, No. 28.—July, 1850. = 
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