The Keweenawan in Minnesota. — Elftman. 143 
Azurite occurs as a blue coating derived from the copper- 
bearing minerals. 
Varieties. The entire gabbro mass consists of minerals 
common in gabbros. These minerals vary in proportion in 
different parts of the rock. There is every gradation from 
the uniform mixture of minerals to masses composed entirely 
of one mineral or in which the other minerals may all be 
present but forming only a small per cent of the whole. While 
the whole rock is included under the general term gabbro, it 
seems that the various mineral combinations which have per- 
sistent characters are of sufficient importance to merit varietal 
names. The separating into varieties is due to forces acting 
at the time of solidification. The principal minerals are nearly 
always present either as essential or accessory, according to 
the variety. 
Normal phase of the gabbro. This phase of the rock 
consists essentially of plagioclase and diallage. The former 
is more abundant. Olivine is nearly always present as an ac- 
cessory mineral. Since the absence of olivine is only excep- 
tional it appears that the division into olivine and olivine-free 
gabbros is wholly superfluous in the Minnesota gabbro. 
Augite and magnetite are never present in large quantities. 
The prevailing texture is coarse grained with the average size 
of the grains between one-eighth to one-fourth of an inch 
across. While the minerals are usually in broad plates, .fre- 
quently they occur as long, narrow individuals similar to those 
found in diabase. All of the mineral constituents can gen- 
erally be identified in the hand specimen. The prevalent color 
is light gray on the freshly fractured surfaces and dull white 
on exposed surfaces. 
Forelle?istein (troctolyte). The olivine in the normal 
phase of the gabbro varies in inverse proportion with the 
diallage. When the olivine predominates over the diallage. 
the other constituents remaining the same, the rock ap- 
proaches the character of a typical forellenstein. In many 
areas of the gabbro, notably in T. 61 N. R. 10 W. and adjacent 
townships, the olivine has nearly replaced the diallage, which 
occurs only in a very subordinate quantity. The rock has a 
spotted appearance and is usually characterized by a banded 
structure. The olivine, when altered, changes to serpentine 
