164 The American Geologist. September, i9co 
may throw all the light possible upon the petrographic ques- 
tions which may arise. 
The following rocks will be studied : 
Olivine gabbro (chapter I). 
Normal diabase (chapter II). 
Olivine diabase (chapter III). 
Plagioclasyte (chapter IV). 
Troctolyte (chapter V). 
Orthoclase gabbro, hornblende gabbro (chapter VI). 
Cordierite noryte (chapter VII). 
Quartz gabbro (chapter VIII). 
Silicoferrolyte (chapter IX). 
In conclusion a final chapter will be devoted to a resume and com- 
parative study of the series. 
Chapter L Olivine Gabbro. 
Although we are studying an area composed largely of 
gabbro of nearly every variety, the normal deep seated gabbro 
of the petrographers is decidedly rare ; indeed, the writer has 
not been able to obtain a typical sample. It occurs*, but only 
locally, and as a variation from the prevalent plagioclasyte, 
where the latter rock is unusually rich in augite. It would 
then consist of a holocrystalline granitoid aggregate essen- 
tially made up of plagioclase and pyroxene in approximately 
equal amounts, with more or less magnetite and a few small 
crystals of apatite. But if strictly normal gabbro is rare, the 
same is not true of normal olivine gabbro, which is the domin- 
ant rock of the main crescent-shaped area which we are study- 
ing. Its geological relations have already been indicated. It 
is a rock of very uniform structure f and composition. Apart 
from a rude bedding, due to localization of tlie pyroxene, or 
olivine, or both in certain bands and their rarity in others, the 
structure is always typically granitoid. The grain varies from 
medium to coarse, but the latter is more frequently seen. The 
rock is always massive and presents in contour a very irregular 
outline of large, low rounded hills, spreading over immense 
tracts of country. The banding or gneissic structure is only 
evident after weathering, the olivine especially altering and dis- 
*See R. D. Irving: Copper-bearing rocks of lake Superior: Geol. 
Wis., Ill, p. 176. 
tCf. W. S. Bayley: The basic massive rocks of the Lake Supe- 
rior region: Jour. Geol., 1893, I, p. 698. 
