170 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
tion,!and are made to include some of the results of Pumpelly’s micro- 
scopic investigations.” 
Of these four groups, the uppermost (Marvine’s ‘‘c,” including beds 1 
to 44, inclusive, of his detailed section) has a horizontal width of 2,700 feet, 
and corresponding thickness of 1,417 feet, extending as far southward as 
the upper falls of Eagle River. It is mostly made up of sharply separable 
diabase flows, nearly every one having its well-marked vesicular or amyg- 
daloidal portion, its pseud-amygdaloidal middle portion, and its compact 
lower portion. The beds, including always these several divisions, run from 
6 to 80 feet in thickness, eighteen ranging from 6 to 20 feet, nine from 20 
to 40 feet, and three from 40 to 50 feet, while one is 62 feet and one 80 feet 
thick. The amygdaloidal or true vesicular portions of these beds range 
from 2 to 10 feet in thickness, rarely exceeding 5 feet. The pseud-amyg- 
daloids run from 5 to 16 feet in thickness, graduating imperceptibly into 
the compact rock below. 
The predominant type of diabase of this group is “a rather fine-grained 
and compactly textured rock, breaking with an irregular to semi-con- 
choidal fracture, and somewhat brittle and elastic. The color is generally 
a dark but dull green, vaguely mottled with purple, the latter often pre- 
dominating, having in it a mottling of green, and occasionally all is green, 
a darker shade mottling a lighter background.” One light-green bed and 
two greenish ones are of exceptional character. The green color of these 
beds is due to the green alteration-product of the augitic ingredient. The 
thin section sometimes shows much of the augite fresh, but always some- 
what altered to the green substance, and in some sections the alteration is 
nearly or quite complete. The specific gravities range from 2.71 to 2.89. 
The amygdaloids carry predominatingly calcite in the amygdules, and next 
prehnite and quartz, the quartz-bearing amygdaloids having often a much 
indurated matrix. This matrix is only rarely similar to that of the under- 
lying compact diabases, ‘‘ being almost always very fine-grained to com- 
pact, sometimes inclined to earthy, sometimes indurated, and generally 
1 Geological Survey of Michigan, Vol. I, Pt. II, pp. 95-140. 
2Metasomatic Development of the Copper-Bearing Rocks of Lake Superior. Proc. Am. Acad., 
Vol. XIII, p. 268 (1878). is 
*Geological Survey of Michigan, Vol. I, Part II, p. 102. 
