TJic Keiveenazvan in Minnesota. — Elftman. 135 
erals. The bands have no general direction of strike and dip. 
appear and disappear at random. Some of the bands 
are composed of the normal phase of the gabbro ; while others 
are composed on the one hand almost wholly of feldspar and 
of the ferro-magnesian minerals on the other. The bands 
vary in thickness from less than an inch to several feet. Each 
band is usually quite irregular, varying considerably in thick- 
ness in different parts. The lines of division between the dif- 
ferent bands usually appear quite sharp and distinct on sur- 
face exposures. A microscopical examination of the different 
bands shows that the mineral constituents arc the same in 
all of them, yet in quite different proportion. The texture of 
the different bands is usually the same and the minerals of 
each band are intimately united with those of the adjacent 
bands, indicating that they were solidified at the same time. 
The banding is similar to that described and illustrated by 
Geikie and Teall in the Tertiary gabbros on the Isle of Skye.* 
4. Frequently considerable areas of the gabbro consist 
chiefly of one mineral. Prominent illustrations of this are the 
feldspar masses in the central part and the magnetite masses 
near the edges and in the eastern end of the gabbro area. 
There are all gradations in size from these large masses to the 
banded and homogeneous rock. The large feldspar masses 
frequently contain patches 10 to 100 feet in diameter, of the 
normal phase of the gabbro. While the outlines of these en- 
closed patches is sharp in places, it usually grades gradually 
into the feldspar rock. In approaching one of the feldspar 
areas the normal massive rock at first shows a spotted and 
then a banded structure, which continues up to the isolated 
mass. The magnetite deposits are surrounded by similar 
conditions. The normal rock becomes banded by a separa- 
tion of the feldspar and magnetite. The latter bands some- 
times increase in thickness to over 200 feet in width or 
widen in places, forming large lenticular and boss-like 
masses surrounded by the normal phase of the gabbro. 
5. The mineral constituents sometimes are arranged with 
their longer axes parallel or lying in one plane. The different 
minerals are proportionately distributed or are separated into 
*Ouart. Jour. Gcol. .Soc. London, 1894, No. 200, pp. 645-660. 
