374 PROCEEDINGS OF SECTION C. 
Andesite. 
27. This rock is a network of narrow plagioclase crystals, the 
meshes of which are filled in by a light-yellow to pale-brown 
augite, and associated with this a considerable amount of ilmenite, 
much converted into leucoxene. The felspars obscured at angles 
of 18° 22’ and 19° 27’ in the zone P.K., thus indicating Labradorite. 
The absence of olivine and the general character of this rock leads 
me to place it among those andesites which are very near to basalt. 
51. One of the most interesting rocks in this collection is a 
holocrystalline compound of augite, amphibol, biotite, plagioclase, 
and orthoclase, the latter taking the place of a ground mass or of 
free quartz in more acid rocks. 
The augite is in colourless hypidiomorphic crystals, which have 
been broken and eroded, and have a somewhat unusually well- 
marked prismatic cleavage. 
The obscuration angles in a section near to M, I observed to be 
42° 52’ in one part of the crystal, and 33° in another, the slice 
being somewhat inclined toward (100) G0 Po. 
There is a good series of augite crystals more or less converted 
into amphibole, the latter being very pale in tint, and having an 
obscuration of 12° 10’ on M, with a pleochroism in pale blue and 
pale yellow. These crystals have a narrow margin of chlorite. 
The augite is also commonly intergrown with biotite, with which 
crystals of magnetite are associated. 
The plagioclase crystals are hypidiomorphic, and are compounded 
according to the albite, Carlsbad, and more rarely the Baveno law. 
Measurements were not altogether satisfactory, but are given 
below. The exit of a bisectrix appeared in a slice approximately 
near the plane K. 
Zone P. K. — Zone P. M. 
17" 59! 31° 43’ 
21° 47’ 
29° 31’ 
These observations point to a felspar of the Bytownite group. 
The orthoclase is in considerable amount, and fills in compara- 
tively large spaces, including other minerals, but especially plagio- 
clase crystals. It thus takes the place of quartz as a residual 
constituent of the magma. 
In only one part of the slice was I able to find a trace of 
cleavage in the orthoclase and in it obscuration was parallel. 
The margin of the orthoclase was in most places edged by growths 
of micro-pegmatite, especially as it would seem where there were 
terminal planes formed. Outside these growths were again small 
amounts of quartz which appear to be of original formation. 
The condition of this rock is surprisingly fresh and unaltered. 
In view of all these particulars I think that it may be placed 
provisionally among the andesites. 
