CHAP THR we 
STRUCTURAL FEATURES OF THE THREE CLASSES OF 
ROCKS OF THE KEWEENAW SERIES. 
The basic rocks; their arrangement in distinct beds or flows.—Division of the finer-grained rocks into 
distinct portions.—The amygdaloids.—Stratiform amygdaloids.—Ashbed amygdaloids.—Notice- 
able absence of beds of true voleanic ash in the Lake Superior region.—The massive basic rocks, 
or so-called ‘‘traps.”—Indications of viscous flow in the basic rocks.—Proof that the bedded 
basic rocks are “‘ contemporaneous.”—Minor undulations in the beds of basic rocks.—Lateral ex- 
tent of the beds of basic rocks.—Lateral extent of the groups of beds.—Thickness of individual 
beds.—Effect upon the topography of the structure of the basic rocks illustrated by occurrences 
on the North Shore; on the South Shore.—Occurrence of cutting masses or dikes.—Character of 
these dikes as seen on the Minnesota coast; these dikes, and those of the underlying slates, fill 
the fissures through which came the bedded basic rocks.—The massive coarse-grained gabbros of 
the Duluth and Brulé Lake regions of Minnesota, and the Bad River region of Wisconsin; are 
they great flows, or do they represent the deep-seated portions of masses from which flows have 
come?—Structural features of the acid rocks.—Granite, augite-syenite and granitic porphyry in 
the Bad River country of Wisconsin and in the neighborhood of Duluth.—The same in north- 
eastern Minnesota, and on the Minnesota coast.—Quartz-porphyry as seen on the North Shore; its 
occurrence at the Great Palisades.—The Palisade porphyry east of Baptism River.—Important 
theoretical bearing of these occurrences of porphyry.—Other occurrences of quartzose porphyry 
on the North Shore.—Quartzose porphyry on the South Shore; at Mount Houghton; in the Onton- 
agon country; in the Porcupine Mountains.—General conclusions as to the origin and relative 
ages of the porphyries and basic rocks.—Structural features of the detrital rocks. 
The basic crystalline rocks make up the greater part of the thickness 
of the series. They occur for the most part in distinct beds, from a few 
feet to several hundred feet in thickness, which, while sharply defined from 
each other, do not commonly possess any subordinate bedding structure, 
though such a structure is at times to be observed, as noted below. 
Often these beds present an easily recognized twofold division, into an 
upper, narrower, amygdaloidal portion, and a lower, compact, non-amyg- 
daloidal portion. ‘This subdivision is one characterizing especially the beds 
composed of the finer-grained diabases, which are, however, much the most 
abundant of the basic rocks. The coarser-grained diabases and gabbros 
are never, so far as my observation has extended, furnished with amygda- 
loids. Of the finer-grained kinds, the olivine-free diabases are perhaps 
somewhat more commonly supplied with amygdaloids than those carrying 
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