PETROGRAPHY. 
69 
In the case of the Bagnorea specimen the phenocrysts, the leucite, the augite and 
olivine (measured together), and the magnetite were readily measured, the detailed 
reductions to weights being found in the formal description. In calculating the 
mode from the norm, after estimation of the augite on the usual basis as to com- 
position, the amount of leucite determined by the microscopic measurements was 
assumed to be correct, since that of the nephelite could not be thus determined 
and a knowledge of the amount of one of the lenads was necessary. 
As the Monte Venere rock contains no nephelite, all the soda was assumed to 
go into the albite molecule; and as it contains some biotite but no olivine, the 
normative olivine was combined with some leucite to form modal biotite. After 
these readjustments and the calculation of the augite of the usual composition, the 
remaining silica was distributed between orthoclase and leucite by the regular 
equations. 
CALCULATED. 
MEASURED. 
Soda-orthoclase Or<Ab 4 
47-7} 
i6.35 
16.9 
10.3 
3-7 
3-8 
0-5 
Vol. %. Sp. gr. 
68.7 X 2.6 = 
20.6 X 2.5 = 
8.0 X 3-3 - 
0.7 X 2.9 = 
2.0 X 5-2 = 
178.6 
51-5 
26.4 
2.0 
IO-4 
Wt.%. 
66.5 
19.2 
IO.O 
0.8 
3-5 
Labradorite, Ab t An 2 
Leucite 
Augite . 
Biotite 
Magnetite . . 
Apatite 
100. O 
IOO.O 
268.9 
IOO.O 
It will be seen that the calculated and measured modes in both cases agree very 
well and that the influence of overlapping has not been serious; indeed in the case 
of the Monte Venere rock the relations of light and dark minerals in the two modes 
are the reverse of those ordinarily obtained, the light and dark minerals being 
respectively less and more in the calculated than in the measured mode. 
As compared with each other the modes of the two rocks are closely alike, 
though there are some differences, as the presence of nephelite and olivine in the 
Bagnorea rock and the presence of biotite in that from Monte Venere. But these 
differences are of slight moment, and may be regarded as negligible, at least for the 
present, when very fine distinctions are inadvisable. 
Compared with the norms the two modes are evidently abnormative, especially 
in the replacement of nephelite (in whole or in part) and orthoclase by leucite, the 
other divergencies being negligible, so that the type may be spoken of as leucite 
alfersalphyro-monzonose-ciminose. 
It is of some interest to compare the fiescolal ciminose [ciminite] with this type 
as regards chemical composition and mode. The chemical differences between the 
two, shown by the lower alumina and alkalis of the fiescolal type, have been adverted 
to above, and it is quite clear that the modal differences, and especially the presence 
of leucite, are largely dependent on these chemical differences. Indeed, in the case 
