282 The Americtin Geologist. November, isiod 
cal composition as described by Boiiney with the unimportant 
exception that the pyroxene does not always show the dial- 
lagic parting. The Minnesota troctolyte is therefore a gab- 
bro consisting of oHvine and plagioclase in nearly equal pro- 
portions. It varies from the normal gabbro merely in that the 
ferromagnesian silicate has crystallized almost exclusively as 
olivine instead of pyroxene. Troctolyte occurs near Duluth, 
near Gabbro lake, and in varous other localities in the gabbro 
area as noted by N. H. Winchell and A. H. Elftman. While 
surrounded by gabbro, the rock has not yet been found grad- 
ing insensibly into the latter; no contacts have been found. 
The structure of the rock is generally massive and coarse, but 
often shows also a certain banded arrangement of the minerals 
on too large a scale to be seen in hand specimens. A bedded 
structure is also reported as quite common.* Its relations in 
the field to the ordinary gabbro have not been fully deter- 
mined, but it is very probable that it is merely a local varia- 
tion of the latter like the plagioclasyte. The color is a very 
dark, mottled, greenish gray; in the background of greenish- 
black olivine, cleavage faces of plagioclase are abundantly 
scattered. The rock weathers to a dark green or rusty brown 
color, which is often mottled with dull, yellowish white decay- 
ing feldspars. 
Under the microscope the texture is seen to be coarsely, 
or medium grained, granitic. The olivine and labradorite are 
in general mutually xenormorphic, but when augite occurs it 
shows a marked tendency to surround the olivine, and may 
even surroiind many grains of varying orientation thus pro- 
ducing the poikilitic texture in the midst of the prevailing 
granitic habit. (See plate XVII, Fig. 9.) Occasionally the oliv- 
ine shows a much rounded crystal form indicating its solidifi- 
cation slightly before the feldspar. Magnetite, as an original 
mineral is remarkably rare while apatite is wanting altogether, 
at least in the sections examined. 
The alteration products observed in this rock are very few 
in number. Bowlingite and magnetite are the only secondary 
minerals at all common; penninite, calcite, and a sericitic min- 
eral, probably muscovite, occur quite rarely. 
*A. H. Elftman: Twenty-third Annual Report, Geol. Nat. Hist. 
Surv. Minn. 1895. p. 224. 
