40 
Metallurgist, was so good as to make a quantitative analysis, not only' 
of the felspars of the specimen No. 445, but also of the rock itself. 
The latter is given in analysis No. I., and the former in analysis No. II., 
below. 
No. I. No. II. 
Silica 
... 6686 
Silica 
... 67-80 
Alumina 
1491 
Alumina 
... 2076 
Ferric oxide ... 
406 
Lime 
3-77 
Ferrous oxide 
365 
Magnesia 
... trace 
Manganese oxide 
trace 
Potash 
... trace 
Lime 
... 3" 25 
Soda 
794 
Magnesia 
Soda 
Potash 
Water at 100° 
tr combined 
1-28 
4-22 
1-49 
0-09 
0-58 
100-39 % 
Sp.gr., 2 - 62. 
100- 27° / Q 
Analysis No. II., when calculated out in the form of a lime felspar 
and a soda felspar, had a surplus of silica which may reasonably be con¬ 
sidered as representing free quartz, attached to or entangled in the felspar. 
Disregarding this, and recalculating the other constituents to 100, and 
again casting as lime and soda felspars, the result was Ab5 An2. The 
specific gravity of the felspar was found to be 2*62, but as this felspar is seen 
under the microscope to be all more or less kaolinized the specific gravity 
is lower than it would be from an unaltered felspar. Further, the kaoliniza- 
tion would result in a loss of lime and soda, alumina and silica, thus disturb¬ 
ing the relative proportions of the felspar constituents, but in what degree 
there is no means of judging. 
Allowing for all this, the constitution of the felspar might be Ab3 Anl, 
which would agree with the optical results. As it stands the felspar would 
be placed in the andesine group, but the observation that the extinction in 
M was + 6° 16' shows that in that instance the felspar is oligoclase. The 
felspar extracted for analysis and the thin slice were both taken from the 
same specimen of rock. 
No. 450. 
This is a good example ot granophyric structure. The rock is composed 
of pseudo-splierulites which, as is usual in such rock structure, consist of a 
central crystal enveloped by micro-pegmatite. In this slice the central 
crystal is either quartz or felspar, and the micro-pegmatite consists of felspar 
and quartz, intergrown in arborescent or fern-shaped forms. These struc¬ 
tures may be pictured as simultaneous intergrowths of a crystal of felspar 
and a crystal of quartz, the component parts of each portion obscuring at 
the same time. If the central crystal is felspar then the surrounding 
pegmatitic extensions of felspar are oriented in accordance with it. So also, 
when the centre is quartz, are the extensions of quartz in optical accord 
with it. 
The conditions under which the rock had crystallized made it difficult to 
obtain satisfactory readings of extinction angles, but those obtained are 
given in the preceding table, being 0° 32' on P and 7° 5' on M. 
