PETROGRAPHY. 
37 
Comparison of the analysis of this leucitic type (I) with that of the non-leucitic 
(III) is of interest. While they are closely similar in the figures for alumina, iron 
oxides, magnesia, lime, and soda, the silica of I is lower by about 2 per cent, and 
potash higher by about 1.6 per cent. These differences are not very great, but they 
are concordant, and sufficient to have determined (it may be concomitantly with 
the physical conditions) the formation of modal leucite in the one and its absence in 
the other. At the same time, it must be understood that modal leucite could have 
formed from the magma of III, as is clear from consideration of the norm, though 
only to a comparatively small extent, limited by the small amounts of normative 
nephelite and olivine which this type shows. 
The analysis in IV has been corrected by me from vom Rath's original one in 
regard to the oxides of iron, titanium, and phosphorus, the corrections being based 
on an analysis of mine of a leucite rock from the same locality. It will be discussed 
later. 
Mode. The measured mode of I was determined both megascopically and 
microscopically, the phenocrysts of leucite having been measured on the surfaces 
of a good- sized hand specimen and those of labradorite and augite, with the con- 
stituents of the groundmass, in the thin section. For the calculated mode the com- 
position of augite was assumed to be that occurring in the Ticchiena albanose, as in 
the preceding case, and that of the biotite to be about equal parts of leucite and 
olivine. This last is not strictly correct, but the error will be slight in view of the 
small amount of this mineral present. 
CALCULATED. 
MEASURED. 
Soda-orthoclase, Or g Ab 7 
39- 6 
17-5 
31-4 
5-3 
2-3 
3-2 
0.7 
Vol. %. Sp. gr. 
42.0 X 2.6 = 
ii. o X 2.7 = 
38.0 X 2.5 = 
5-4 X 3.3 = 
2.0 X 2-9 = 
1.6 X 5.2 = 
Wt. %. 
109.20 41.1 
29.70 II. 2 
9S- 35-7 
17.82 6.7 
5 . 80 2.2 
8.32 3-1 
Labradorite, AbjAn 2 
Leucite 
Augite 
Biotite 
Magnetite 
Apatite . . . . . 
IOO.O 
IOO.O 
265.84 loo.o 
^ I It will be seen that the correspondence is fairly close for most of the constitu- 
ents, especially when the very fine grain of the groundmass is taken into considera- 
tion. The only marked discrepancies are in the amounts of labradorite and leucite, 
which are respectively lower and higher in the measured mode. The former is prob- 
ably due in part to the presence of some labradorite inclusions in the leucite pheno- 
crysts, which were not reckoned in, and also in part to the possible presence of some 
lime-soda feldspar among the groundmass laths. The measured leucite must, of 
course, be diminished by the amount of labradorite which it includes, as well as by 
the small quantities of augite and glass inclusions, whose exact amount could not 
