2 THE JOURNAL OF GEOLOGY. 
The Olivimtic Varieties—The few olivine-bearing granulitic 
gabbros studied are from places but a very short distance south of 
the north limit of the gabbro area, and not far north of localities 
where the normal gabbro occurs in its usual form. They appear 
to be intermediate in character between the gabbro and the typ- 
ical granulitic rocks. Their structure is not purely granulitic, 
for the pyroxene in them is in polysomatic masses rather than in 
individual rounded grains, although in the same section there 
may be found in addition to the polysomatic masses a few isolated 
grains of augite and some compact plates of the same mineral. 
The olivine is in fairly large, colorless or light yellow, irreg- 
ular grains, whose cleavage cracks are marked by grains of mag- 
netite and masses of limonite. Magnetite also occurs as small 
dust-like particles in their peripheral portions. Green earthy 
decomposition products surround many of the fresh olivines and 
extend into their interiors along irregular cracks, and out into the 
minerals that are contiguous to them. A feature of great signifi- 
cance in connection with the olivine is the fact that many of its 
grains are surrounded by narrow rims of augite, just as are the 
olivine grains inthe normal gabbro.* Since this method of asso- 
ciation of the two minerals is so characteristic for the gabbro, 
which is undoubtedly an igneous rock, there can be no question 
that the olivine granulitic rocks, though bedded, are closely 
related to the gabbro, and like this rock are also igneous, and 
are not metamorphosed sediments. Unfortunately, most of the 
granulitic rocks contain no olivine, so that the application of this 
means of identifying them as portions of the great gabbro is very 
limited. 
The diallage appears to be next in age to the olivine. It is 
in small rounded grains, either isolated or in aggregates so united 
as to form large masses. The individual grains of the aggregates 
are often separated from each other by narrow seams of limonite, 
or by small grains of magnetite, and in their midst is not infre- 
quently a grain of olivine. The mineral is nearly colorless. It 
possesses the normal prismatic cleavage of augite, and in addi- 
*See this Journal, Vol. I., p. 702, Fig. 1, and p. 706, Fig. 3. 
