é 
SUMMARIZED SECTION OF EASTERN PART OF KEWEENAW POINT. 187 
Thickness in feet. 
The Greenstone Group; made up of relatively heavy beds, without amygda- 
loids, of rocks for the most part relatively coarse-grained; these belong 
mostly to the coarse-grained olivine-free diabases and gabbros and to the 
luster-mottled melaphyrs, or fine-grained olivine-diabases, the Green- 
stone at the base of the group being of the last-named class............ 1, 200 
The Sub-Greenstone Group, in which all of the fissure-vein mines are working; 
having at top a thin conglomerate, the equivalent of the “ Allouez” and 
“Albany and Boston” conglomerates in the Portage Lake district ; com- 
posed of fine-grained diabases, with not very strongly developed amygda- 
LOS QLSCHUN oaadiatse ieee BOGAN ENE e See cetera sear Cet ear adel a a 1,600 
The Central Valley Beds; the layers not well exposed, but evidently chiefly 
fine-grained diabases and amygdaloids, with a number of thin porphyry- 
conglomerates, in all respects like the overlying group; about........... 5,540 
The Bohemian Range Beds ; made up chiefly of diabases and melaphyrs in all re- 
spects like the higher layers, and including some of the usual porphyry- 
conglomerates; but also in part made up of quartziferous porphyry, fel- 
site, and non-quartziferous porphyry, and coarse-grained orthoclase-gab- 
LORDS TD DU CVO See ted 8 a ear ee ee Ur ear en OE Samer Lae 10,000 
THRE, Vigo ec RAReae Re fe nice cree ne ee a a ... 26,000 
Southwestward along the strike from the Eagle River type section 
above described, both the topographical and geological characters of this 
section soon change. By the time the Gratiot River is reached the several 
ridges have all merged into one broad swell, the median valley disappearing. 
The geological changes of importance are: (1) A general thinning of the 
series, due in some considerable measure to the nearly complete disappear- 
ance of the group of coarse-grained diabases and luster-mottled melaphyrs 
which, under the name of the Greenstone Group, has been described as so 
prominent a feature in the geology of the eastern part of Keweenaw Point: 
This thinning, however, is by no means confined to the Greenstone Group, 
the rocks above and below thinning as well. In the Eagle River section, 
between the slide at the base of the Greenstone and the base of the Great 
Conglomerate, there is a thickness of some 4,000 feet, while the equivalent 
beds at Portage Lake cannot be more than 2,300 feet thick. (2) A consid- 
erable increase in the amount of lakeward dip, which at the Allouez mine, 
Sec. 31, T. 57, R. 32 W., reaches 46°; at the Albany and Boston, See. 8, 
T. 55, R. 33 W., 52°; and at the mines about Portage Lake, 55°. (3) The 
fault line, or contact with the Eastern Sandstone, is now at a much higher 
horizon in the trappean series than further east; so much so that in all of 
