LITHOLOGY. 35 
Section IL—BASIC ORIGINAL ROCKS. 
In his paper on “The Metasomatic Development of the Copper-bearing 
Rocks of Lake Superior,” published in 1878,’ Pumpelly first showed the 
true nature of the prevalent basic rocks of the copper region of Michigan. 
His work was chiefly on specimens from the section displayed on the lower 
Eagle River, Keweenaw Point. Subsequently (1880), the same geologist 
published? the results of an examination of a suite of specimens collected 
for the Wisconsin Geological Survey by Messrs. M. Strong, E. T. Sweet, 
and myself in the districts severally under our charge. These Wisconsin 
rocks he was able for the most part to throw into the same groups whose 
existence he had already determined in his Michigan work, the only im- 
portant addition being one or two varieties of true gabbro, now first proved 
to exist in the Lake Superior country. These gabbros were further de- 
scribed in the same volume by Mr. A. A. Julien, and in some detail by 
myself. 
At the close of the investigations of the Wisconsin Survey the list of 
the basic rocks of the Keweenaw Series stood as follows: gabbro, including 
gabbro proper, olivine-gabbro, uralitic gabbro, and an orthoclase-bearing 
gabbro; diabase, including the ordinary, prevalent, fine-grained type, one 
or two coarse-grained varieties, a type designated as “ashbed” diabase, a 
pseud-amygdaloidal diabase, and true diabase-amygdaloids; and melaphyr, 
including melaphyr proper, as also its pseud-amygdaloidal and true amyg- 
daloidal phases. The names gabbro, diabase, and melaphyr were given in 
accordance with the usage of H. Rosenbusch, who has classed the pre- 
tertiary plagioclase-augite rocks as indicated in the following scheme:* 
I. GRANULAR. 
a. Plagioclase-augite = diabase. 
b. Plagioclase-augite-olivine = olivine-diabase. 
II. PoRPHYRITIC, containing more less of an insoluble base. 
a. Plagioclase-augite = diabase-porphyrite. 
b. Plagioclase-augite-olivine = melaphyr. 
1 Proceedings of the American Academy of Arts and Sciences, Vol. XIII, pp. 253-309. 
2 Geology of Wisconsin, Vol. II, pp. 29-49. 
3Microscopische Physiographie der Massigen Gesteine, von H. Rosenbusch. Stuttgart, 1877, pp. 
317, 458. 
