PETROGRAPHY. 
2 5 
lastonite was taken as a basis, which would give a composition approximately like 
that of most aegirite-augites, the rest of the hematite being placed with the ores. 
The CaO of the wollastonite takes up, then, just the amount of A1 2 O 3 set free from 
nephelite by the transfer of Na 2 O to acmite, to form anorthite (3.3 per cent) which, 
with that calculated normatively, must exist in the feldspar. A small amount of 
nephelite is necessarily present, but this would easily escape observation, owing to 
CALCULATED. 
MEASURED. 
Soda-orthoclase, 
Nephelite 
Sodalite 
^Egirite-augite . . 
Ores 
:!! 
3 
3-9 
4.0 
Vol.% Sp.gr. 
84.7 X 2.6 
4-9 X 2.2 > 
6.6 X 3.3 
3-8 X 5-2 
IOO.O 
Wt.% 
81.5 
3-8 
7-7 
7-Q 
282.6 100.0 
230.2 
10.8 
21.8 
19.8 
the fabric of the groundmass. Allowing for these readjustments, the average 
composition of the alkali-feldspar would be Or 76 Ab 75 ; that is, almost exactly 
OrxAb,. 
In the measured mode the nephelite has been measured with the feldspar, and 
the amount of these is seen to be rather less than the calculated amount. On the 
other hand, the measured amounts of pyroxene and ores, the colored minerals, are 
rather higher than those calculated by the same figure, these for sodalite being 
almost identical. This discrepancy, though not serious, may be attributed to the 
overlapping of the small mineral grains, -and a consequent greater apparent amount 
of the colored ones, a point we shall have to notice later. 
The mode of the blocks from Monte Vico was also calculated and measured 
with the following results. The process of calculation was very similar to the pre- 
CALCULATED. 
MEASURED. 
Orthoclase, Or 2 Ab t 
% 
Si.jl 
5-45 
2.1 
4-5 
4.2 
0.6 
Vol. % Sp. gr. 
85.3 X 2.6 = 
2.9 X 2.3 = 
8.6 X 3-3 = 
2.7 X 5-2 = 
o-5 X 3-5 = 
221.8 
6-7 
28.4 
14.0 
i-7 
Wt.% 
81.4 
2.4 
10.4 
5-2 
0.6 
Nephelite 
Haiiyne 
^Egirite-augite 
Ores 
Titanite 
99-5 
IOO.O 
272.6 
IOO.O 
ceding. The measured amount of titanite was assumed to be correct, and the FeO 
thus set free from normative ilmenite placed in a diopside molecule, the extra lime 
being taken from normative wollastonite. A small amount of aegirite was arbi- 
trarily calculated, as in the last case, and an amount of anorthite corresponding to 
this calculated, the total amount of this being 3.6, which goes into the feldspar. 
