988 The Crystalline Rocks of the Northwest. [October 
classed as older than the series which has latterly been designated 
Huronian, and by others styled igneous and local, it has, by 
Brooks, been placed with that series and denominated “the young- 
est ” of the Huronian strata, though no such rocks had ever before 
been mentioned as pertaining to the Huronian. By Irving it has 
been made the base of his Kewenawan. By Hunt it has been paral- 
lelized with the Montalban. It includes, in my opinion, the fels- 
ites and porphyries which have been styled Arvonian, and itis 
very certain that in many places it has passed for typical Lauren- 
tian. The gabbro is very generally admitted to be of eruptive 
_ origin, and in its great development in Canada it was once styled 
Upper Laurentian, and later was known as Norian. While the 
gabbro is certainly eruptive, the associated granite and gneiss 
exhibit evidences of being metamorphic in their nature. In 
Northern Minnesota this horizon of granite is characterized by 4 
red color and it has an aggregate chemical composition almost 
identical with that of some of the associated felsites. The mag- 
netite of the gabbro is often highly titaniferous and so abundant 
that the rock has attracted attention as an iron ore. The gabbro 
does not always appear where the granite is present, but extensive 
areas of granite are spread out without any sign of variation, 10- 
terruption or alternation with the gabbro. In other places these 
two rocks are intricately and intimately mingled both horizontally 
and perpendicularly ; but the gabbro can be considered in general 
as the underlying formation. Both these rocks seem to, have 
been molten, and simultaneously so, in some places ; but ın the 
great mass of the red, granitic rock, there is a gneissic structure, 
and in its finely crystalline state, when it seems to vary to felsite, 
it exhibits a laminated structure which is evidently due originally 
to sedimentation. Along these laminations, and coincident e 
them, is a finely lined striation which exhibits the “stre ; 
structure, sometimes appealed to, to show the igneous nature an 
origin of the rock. These felsites are occasionally arenact™ 
with ir regularly rounded or sub-angular quartz grains, and em 
times are porphyritic with quartz and orthoclase. Veins of a 
granite intersect the gabbro, and the gabbro surrounds isola 
masses of the granite. Transported, boulder-like masses of i 
_ are found embraced in a common paste among the later igneo : 
outflows of the Cupriferous, where their existence is aS pai 
puzzle as that of pebbles of red felsite and quartz-porphy!y 8 
red conglomerates. This red granite, so far as I have obs 
