8 
MUSEUM BULLETIN NO. 30 . 
feldspar is usually very white, the augite more diopsidic in 
appearance than ordinarily. In some varieties there exists a sub- 
parallel arrangement between the feldspars and the augites, 
which gives the rock a*peculiar and conspicuous columnar tex- 
ture, but in others this is absent and the two minerals fq|rn an 
interlacing network. 
Type C is characterized by the occurrence of the augite in 
large crystal's and crystal aggregates, which enclose the feldspars 
poikilitically. These clumps of ferromagnesian mineral are more 
resistant to erosion than the surrounding more feldspathic parts, 
so that the weathered surface acquires an uncommon, embossed 
appearance. In one variety the clumps of ferromagnesian min- 
eral have an average diameter of 4 to 5 mm., in a second variety 
of from 2 to 3 cm. 
Augite Gabbro. 
The augite gabbro contains no olivine, but is otherwise 
identical, macroscopically and microscopically, with the normal 
olivine gabbro. The unusual columnar texture, type B, that 
distinguishes some of the varieties of olivine gabbro, was not 
found in any of the augite gabbros; but otherwise most of the 
variations in grain and composition found in the one can be 
duplicated in the other. The composition of the constituent 
minerals is also much the same in both. Most of the feldspar is 
bytownite, near Ab 20 An 80 , but whereas in the olivine gabbros the 
range of composition extends only to Ab 40 An 60 , in the augite 
gabbros it attains Ab 60 An 40 . The augite, so far as cguld be 
determined by microscopic means, is the same in both rocks. 
Gneissic Phases . 
Both, the olivine and augite gabbros frequently assume 
gneissic structures and textures. These may be found here and 
there over the whole surface of the intrusive, but are much more 
numerous near the periphery than towards the centre. Alto- 
gether, gneissic rocks may form 2 or 3 per cent of the whole sur- 
face. The most common structure of this sort is primary gneissic 
