374 HENRY S. WASHINGTON 
rocks, shows that it is largely composed of the normative diopside. 
With this there enters also a smaller amount of the normative anorthite 
to furnish the alumina, and of normative magnetite to furnish the 
ferric oxide, both of which are present in the augite. To satisfy the 
demands of the normal augite formula, which may be stated as 
mCa(Mg,Fe)Si1,0,+n(Mg,Fe)(Al,Fe),SiO., ferrous oxide and mag- 
nesia must be supplied from normative olivine, which will take up as 
much silica again as it contains to form a metasilicate molecule. 
The ferrous oxide of the normative magnetite will also demand its 
equivalent of silica to conform to the augite formula. Such, at least, 
was the set of readjustments which were found to be demanded in 
the case of the Italian volcanic rocks, the augite of which had been 
analyzed, and it is probable that the same or a very similar set would 
hold good in others. It will be seen that these readjustments are in 
the direction of the abstraction of silica from the magma by the early 
crystallization of augite, so that, while they may be slight in these 
rocks which are persalanes or dosalanes, yet the tendency of the crystal- 
lization of augite from a magma would be to favor the later formation 
of leucite. The influence of the early crystallization of either horn- 
blende or biotite would be quite analogous; but, as they seldom occur 
in connection with leucite, they need not be discussed. 
Examining the modes of the abnormatively leucitic rocks plotted 
on Plate II, it will be found that the great majority of them (belong- 
ing to domalkalic or alkalicalcic rangs) carry considerable soda- 
lime feldspar, much of which has crystallized prior to the leucite, 
so far as may be gathered from the published descriptions, and as 
may be inferred from the usual order of crystallization. The sodic 
character of the alkali-feldspar is less easily demonstrable in most 
cases, but in the Italian rocks studied by me it was shown to be invari- 
ably decidedly sodic, ranging from Or,Ab, to Or,Ab, in the modally 
leucitic rocks, the composition in most cases being about Or,Ab,, 
and from Or,Ab, to OrgAb, in those free from leucite, the average 
composition being about Or,Ab,. ‘The more highly sodic character 
of the alkali-feldspars in the modally leucitic rocks is thus quite . 
marked. Thisis perhaps best seen in the case of two arsal monzonoses. 
That from Poggio Cavaliere contains no modal leucite, and its alkali- 
feldspar has the composition Or,Ab,; while that of L’Arso contains 
