134 The Ametican Geologist. September. i898 
bodies of the two, and the red rock dikes run for several miles 
from the parent mass. In the vicinity of the isolated red 
rock bosses the gabbro is usually cut by dikes, from a few 
inches to ten feet in width, radiating from the central mass 
and increasing in number as the boss is approached. Gabbro 
dikes are not known to cut the red rock. In many localities 
large masses of gabbro and red rock are in contact, but with- 
out any evidence of their relative age. All the evidence, how- 
ever, agrees that the rod rock acts as an intrusive toward the 
gabbro, and that makes the latter an older rock than the red 
rock. 
The Beaver Bay diabase is everywhere separated from the 
gabbro and so far as their relation to the other members of the 
series shows, these two may be regarded as contemporaneous 
rock masses. The Temperance River flows are largely separ- 
ated from the gabbro. West of Duluth these flows are un- 
conformably above the gabbro. The later diabase cuts the 
red rock and gabbro. The evidence thus shows that the 
gabbro is above the Animikie or Upper Huronian, and that it 
is the lowest or oldest member of the Keweenawan series. 
Structure. The gabbro forms an enormous crystalline 
mass varying in structure from a homogenous massive to a 
banded and apparently bedded rock. The tendency to segre- 
gation into large masses composed chiefly of one or only a 
part of the minerals is well brought out. The various stages 
of segregation are clearly defined and appear in the following- 
order: 
1. A homogeneous rock in w^iich all of the minerals 
present are uniformly distributed. 
2. The feldspars, the ferro-magnesian and iron-bearing 
minerals become separated into aggregates giving a peculiar 
spotted appearance to the rock. The diallage and olivine 
form clusters of grains sometimes radially arranged, these 
clusters vary in diameter from one to four inches. The 
plagioclase which makes up the greater part of the rock also 
fills the spaces between the diallage and olivine grains, and 
thus serves as a matrix of these minerals. 
3. In many places the gabbro possesses a marked band- 
ing. This consists of alternating layer§ of different mineral 
composition, rendered conspicuous by the color of the min- 
