Gabbroid Rocks of Minnesota. — Winchell. 363 
digestion by the gabbro magma of a slight proportion of a 
rock more acid and potassic. 
In exterior characters the rock is at once distinguished by 
the red color of the orthoclase — a color due to numberless in- 
cluded hematite particles. It has a columnar structure only 
rarely found in the ordinary gabbro. The hornblende occa- 
sionally gives the rock a dark green color. The texture is still 
granitic, and usually of very coarse grain. 
The orthoclase is the characteristic mineral, distinguish- 
ing the rock from the normal gabbros. Tlie pyroxene has a 
remarkably small optic angle ; it has in many cases been 
transformed to hornblende. Magnetite is very rare as a pri- 
mary constituent, occurring ordinarily only in very small 
grains. As a more recent mineral it is commoner in large 
crystals and masses. Apatite occurs both as very large crys- 
tals, usuallv corroded and full of inclusions, and often biaxial, 
and as very minute well formed crystals, uncorroded and 
showing few inclusions. The latter are considered to repre- 
sent the recrystallized phosphate derived from the corrosion 
of the 'former. This rock has also been termed hornblende 
gabbro, but it is clear that the hornblende is wholly derived 
from the alteration of the pyroxene. The alteration has pro- 
gressed more rapidly in the orthoclastic type than in the nor- 
mal gabbro on account of the presence of innumerable inclu- 
sions in the former, rendering it highly porous. 
Finally, one of the most remarkable types of the series is 
the cordierite fioryte which is considered to be an endomorph- 
ic state of the gabbro. The field relations indicate it, and the 
microscopical and chemical study add many arguments in 
favor of this theory. 
The rocks which have Ix'cn styled ''mr.scovadyte" by the 
Geological and Natural History Survey of Minnesota, litho- 
logically often belong to a series which forms the contact 
zone, and they are terms of passage from the gabbro on one 
side, to the schists on the other. These contact rocks are 
nearly always very fine grained. Beginning with the unmodi- 
fied gabbro, the important types known to occur frequently 
are : gabbro, noryte, cordie rite noryte, "greenstones."* 
* This is the same as die scries found by Lacroix at Pallet. 
In Minnesota there exist all the intermediate types, e. g. between 
gabbro and noryte there are hy])cr;thene gabbros, and augitc norytes. 
