FORMATION OF LEUCITE IN IGNEOUS ROCKS 381 
The normatively or modally leucitic rocks plotted in the diagram are 
distributed thus: 
sPhose. with:normative-and modal-leucited 74. 5 °.- = 3... . ‘40 
Those with normative but not modal leucite (vitreous) . . . . . . aI 
Those with normative but not modal leucite (holocrystalline) . . . . 29 
‘chosen wath, modal’ but abnormative leucite... =... 3 3 «. =. «- 43 
Reckoning the vitreous rocks with the first group, it will be seen 
that those which do not contain leucite when they should, and those 
which do contain it when they should not, pretty nearly balance each 
other, though the former are somewhat in the minority. They 
correspond, however, sufficiently well to allow the assumption for 
our purpose that the actual number of modally leucitic rocks corres- 
ponds with the number of those which should be leucitic according 
to our theory. 
Now, taking the number of analyses to represent the relative 
amounts of the various kinds of igneous rocks, which is admittedly 
not strictly accurate, but sufficiently so for our purpose, and in any 
case being the only statistical basis we have, we find that all the 
analyses of igneous rocks to be found in Roth’s Tabellen and in my 
Collection, as well as in the analyses published since and collected 
by me, number altogether 6,333. Of these, 278 are called leucitic, 
that is 4.41 per cent., while 6,055, or 95.59 per cent. are non-leucitic. 
While these figures are not of very great significance, on account of 
the unsatisfactory character of the data on which they are based, 
yet they are of some interest, and, taking into account the general 
characters and the average of igneous rocks, which would tend to 
lower very much the relative amounts of leucitic rocks, they are 
quite in harmony with our theory. 
We may briefly summarize our comparison of the actual and the 
theoretical occurrence of leucite as follows: 
The vast majority of igneous rocks conform to theory, based on 
the superior affinity of potash for silica, in that those whose norms 
are free from leucite are for the most part free from modal leucite, 
and the loci of nearly all of them fall outside their respective leucitic 
areas; while those whose norms are leucitic are very commonly modal- 
ly leucitic also, and their loci all fall within their leucitic areas. 
As regards modal leucite, there are certain exceptions which, 
