282 The American Ge oh cist. 
Bytownite 
II 
48.03 
33-43 
16.28 
I 
SiO^ 
48.12 
TiOg 
Al.O, 
34-54 
Fe.Oa 
0.36 
FeO 
MnO 
CaO 
16.30 
MgO 
trace 
K^O 
0.15 
Na^O 
1.91 
HgO 
none 
(Ignition) 
2.26 
ist. 
N. 
ovember, 1890 
H( 
arnblende 
A 
III 
IV 
41.40 
39-58 
trace 
15-39 
14.91 
7.01 
4.01 
7.17 
10.67 
trace 
trace 
12.53 
11.76 
10.31 
13.06 
1.56 
0.62 
3.58 
2.87 
0.81 
2.79 
Total 101.38 100.00 99-76 100.27 
.Sp. gravity 2.731 2.735 3,i8ati5°C 3.217 to 3.222 
I. Bytownite from the anorthosyte of South Sherbrooke, Ontario. 
II. Bytownite, AbiAn4, theoretical percentage given by Zirkel 
(Petrographie, page 221). 
III. Hornblende from the anorthosyte of South Sherbrooke. 
IV\ Hornblende from a hornblende gabbro, ijuoted by Rosenbusch 
(Elemente der Gesteinslehre, page 148). 
It will be seen that there is a reniarkalily close agreement 
between the theoretical composition of the byto.wnite, x\bi 
An4, as regards the percentages of the silica, lime and alkalis 
and that of the feldspar from Sotith Sherbrooke. The pro- 
portion of alumina in the latter is, however, over one percent 
higher than that of the theoretical percentage. This may be 
dtte to the presence of a small amount of corundum embedded 
in the feldspar in a very fine state of division as this mineral 
occurs, as has been stated, in parts of the rock. None of it 
was identified, however, either by megascopic or microscopic 
examination in the specimen analyzed. In thin sections, under 
the microscope, the feldspar is seen to contain ntmierous crys- 
tallite-like inclusions. It is possible that some of these are co- 
rtmduni. 
The hornblendic constituent of the anorthosyte is seen by 
the analysis to have the composition of the typical variety or 
what is called common hornblende. 
