Gabbroid Rocks of Minnesota. — Wi?ichell. 1 5 7 
The most recent study of this part of Minnesota has ap- 
peared within the last two years in the form of volumes IV and 
V of the final report of the geological and natural history sur- 
vey of Minnesota by the state geologist, Prof. N. H. Winchell. 
The views therein presented are those reached after a long and 
detailed as well as comprehensive study, not only in the field 
but also in the laboratory, of the whole rock series found in 
northeast Minnesota. These conclusions will be accepted 
here. To commend, or to criticize them are equally beyond 
the purposes of this paper. 
The conclusions of the state geologist of Minnesota dififer 
radically in one important respect from those of all previous 
writers. The gabbros have been considered heretofore to be of 
deep-seated eruptive origin. On the other hand Prof. N. H.Win- 
chell presents evidence to show that they have been derived 
from the fusion of the great terrane which has been called 
"greenstone" in the Archean, and that they vary in structure 
and composition because the greenstones originally varied. 
The gabbro still retains in some places the original bedding of 
the greenstones, which are themselves generally very basic 
rocks, but become acid by the acquirement of elastics or de- 
tritals. 
According to Prof. N. H. Winchell the evidence that the 
gabbro has been formed at the expense of the greenstones is 
of two categories : — 
1. All the minerals of the gabbro recur in the noryte 
(transition type between the gabbro and the greenstones) or 
are represented by their homologues. But in the noryte they 
present a very interesting texture and frequently enclose 
quartz in poikilitic fashion. Cordierite, fayalite, diallage, 
augite, hypersthene and labradorite are abundant; sometimes 
also enstatite, magnetite, biotite and quartz, more rarely antho- 
phyllite or cummingtonite. All these minerals have been 
formed in peculiar physical conditions which have caused them 
to take the globular form, so frequent in secondary minerals 
produced under the influences of metamorphism. 
2. In certain localities the gabbro passes by insensible 
degrees to the muscovadyte, or noryte, and thq latter to green- 
stone. These passages occur both in normal masses and in 
cases where the rock is conglomeratic. In the latter case the 
