LOWER DIVISION OF THE KEWEENAW SERIES. 155 
est kinds have been observed only at the base of the series (Bad River 
gabbros; Duluth gabbros). (2) Very heavy bedding is also much more 
common at low horizons; and this statement affects both coarse-grained and 
fine-grained kinds (e. g., the fine-grained diabases of the Duluth Group of 
the North Shore). (3) The amygdaloidal texture is more frequent and 
more highly developed at high horizons than at low; the thinner beds gen- 
erally having the most strongly developed amy gdaloidal or vesicular por- 
tions. (4) As to the distribution of the different kinds of basic rocks, the 
fine-grained, olivine-free, or “ordinary” diabases affect very decidedly the 
higher horizons (¢. g., Keweenaw Point), though occurring throughout ; 
olivine-bearing kinds, both the coarse-grained gabbros and fine-grained, 
luster-mottled kinds, are as decidedly more common at low horizons, though 
as before not restricted to them (e. g., the Greenstone Group of Keweenaw 
Point); the ashbed-diabases and diabase-porphyrites are also very 
much more common at low horizons (e. g., Lester River Group; Duluth 
Group, &c.); and the same is true of the orthoclase-gabbros.- (5) Of the acid 
rocks all kinds affect especially low horizons, rarely reaching above the 
middle of the Lower Division. The porphyries of the region between the 
Ontonagon and Bad rivers on the South Shore seem to be an exception to 
this rule; but in this case their appearance at so high a horizon may be 
due in some measure to the thinning out of overlying beds. A more cer- 
tain exception to the rule is probably to be found in the case of the red fel- 
site of the islands of the harbor on the south side of Michipicoten Island. 
(6) Detrital beds, chiefly porphyry-conglomerates and red sandstones, oc- 
cur throughout the series, having been seen all the way from the base to 
the summit, but they are rare in the lower third of the series, and as a rule 
increase in thickness and frequency towards the top, only one instance, to 
be noted hereafter, being known of a heavy bed at a low horizon. 
The coarse gray gabbros so largely developed in the Bad River coun- 
try of Wisconsin, at the base of the series, present the appearance of a cer- 
tain sort of unconformity with the overlying beds. These gabbros, which 
lie immediately upon the Huronian slates, form a belt which tapers out rap- 
idly at both ends, and seems to lie right in the course of the diabase belts to 
the east and west, since these belts, both westward toward Lake Nu- 
