FORMATION OF LEUCITE IN IGNEOUS ROCKS 367 
lite, relatively less is left for the potash. The latter enters into the 
modal nephelite, which, as we know, always contains considerable 
potash and apparently has a rather complex molecule. Nephelite 
is present in very large amounts in these rocks, so that its mass action, 
as well as that of the abundant and complex pyroxene (which often 
contains some potash), may reasonably be considered to come into 
play in capturing the potash and preventing the formation of leucite. 
It is highly probable, also, that the physical conditions enter in as 
factors, since the rocks just discussed are all intrusive, while we 
find some with very similar chemical characters (as Nos. 99, 162, and 
163) which have solidified at or near the surface and which contain 
much modal leucite, in spite of their high content of soda. 
There are also some melilite basalts (Nos. 146, 157, 179, 186, 187; 
188, 194 ,and 195), in which glass is present either in very small 
amount or not at all, so that the absence of leucite from these effusive 
rocks cannot be ascribed to incomplete crystallization. ‘This is to 
be attributed rather to the mass action of the abundant, complex 
melilite, which always contains some potash; to the comparative 
richness of these rocks in soda, and the consequent forcing of part of 
the potash into the abundant nephelite; and in some instances to 
conditions which favored the stability of biotite, as in the alnoal 
types (alndites). . 
Let us now examine the exceptions of the other sort, those with 
abnormative modal leucite. ‘These are exceptional in that, although 
the magma contained sufficient silica to permit the entrance of all 
the potash into orthoclase, as an actual fact this constituent did not 
take all of its theoretical quota of silica, but was content with less, 
forming leucite in whole or in part. 
Examining the norms of these rocks, it is found that, with a few 
exceptions to be discussed later, they are all deficient in silica, norma- 
tive nephelite and olivine being present in every case. Therefore 
the silica liberated through the formation of leucite instead of ortho- 
clase by the potash could form either albite from the normative 
nephelite, or pyroxene from the normative olivine, or both. We find, 
however, that olivine is present modally in nearly all these rocks, 
and that they contain almost constantly abundant albite, either in 
soda orthoclase,or in plagioclase, or in both of these. Furthermore, 
