TJic Keweenawan in Minnesota. — Elftman. 147 
order: Apatite, magnetite, olivine, hypersthene, augite. dial- 
lage. The plagioclase seems to be contemporary with the 
other minerals. The magnetite, while beginning to form 
before the olivine, did not cease to form until some time after 
the latter began. Since the olivine is usually surrounded by 
the pyroxene, its crystallization was apparently completed be- 
fore the remaining minerals were formed. Hypersthene next 
began to form and continued after the augite began. The 
augite usually surrounds the former, but small grains of au- 
gite are also enclosed by some of the hypersthenes. The py- 
roxenes and plagioclase probably began to form about the 
same time. So far as the relation between the two shows, it 
seems that the plagioclase whose outlines depend upon the 
irregular anhedrons of pyroxene is younger than that part of 
the pyroxene, and that the plagioclase enclosed by the ophitic 
areas of pyroxenes is older than the latter mineral. There is 
no evidence that there was a cessation in the crystalization of 
the rock, but it seems that the process was continuous. 
Alteration. — The alterations of the minerals are as follows: 
The pyroxenes change directly into hornblende, and biotite. 
The hornblende also changes to biotite. Olivine forms ser- 
pentine. Plagioclase is replaced by quartz and kaolin. Biotite 
forms a reaction product between plagioclase and magnetite. 
Chlorite, epidote, iron oxides, kaolin, leucoxene, serpentine, 
calcite and the copper carbonates are decomposition products. 
The alteration of the mineral components of the gabbro, 
locally sometimes proceeds far enough to form a rock whose 
mineral composition is quite diliferent from that of the original 
gabbro. An example of this is found at places along the 
southern edge and in the eastern end of the gabbro area. The 
rock is originally composed essentially of plagioclase, ortho- 
clase and pyroxene. The pyroxene is changed to hornblende 
and biotite. The feldspars are in part decomposed into kaolin 
and silica. Quartz replaces the decomposition products. The 
rock is then composed essentially of plagioclase, orthoclase. 
quartz, hornblende and biotite. and becomes a granite in com- 
position. In the field it is sometimes difficult to mark the 
boundary between this altered phase of the gabbro and the 
augite syenite of the red rock member. 
Contact Phenomena. — The gabbro presents remarkable 
