386 HENRY S. WASHINGTON 
If leucite is not present in the norm, we should not expect to find 
it in the mode, especially if the rock is intrusive. But if the magma 
has solidified under effusive conditions, is rather low in silica and 
high in potash, and if it contains considerable salic lime and soda, 
leucite is apt to form. The crystallization of soda-lime feldspar, 
and less often of soda-orthoclase or analcite, is a prerequisite to the 
formation of leucite in such cases, and the tendency to its formation 
will be the greater the higher the rock is in potash, though the presence 
of considerable salic lime and soda may bring about the crystalliza- 
tion of leucite even if the amount of potash is small. It follows 
from this that soda-lime feldspars will be frequent concomitants of 
leucite; and this we find to be actually the case. 
The presence in the norm of such mineral molecules as nephelite 
or olivine, which are not fully silicated, is a necessary condition for 
the formation of modal leucite, if there is none in the norm; that is, 
the magma must be deficient in silica. Of these minerals, nephelite 
is the most important, modal leucite being usually formed by a read- 
justment of silica involving the taking-up of silica by normative nephe- 
lite to form modal albite, which usually enters the soda-lime feldspars. 
Olivine is much less prone to take silica away from potash, but 
may doso. The crystallization of alferric minerals, especially augite, 
also favors the formation of leucite by abstracting silica from the 
magma. 
Effusive conditions tend to favor, and intrusive conditions tend 
to check or wholly prevent, the formation of leucite in magmas from 
which it is chemically possible for it to form. It is because of this 
that augite occurs more often with leucite than do hornblende or 
biotite. In general, the leucitoid mineral formed under intrusive con- 
ditions is the so-called pseudo-leucite, which contains a very consider- 
able amount of soda, and which consists of an intimate mixture of 
orthoclase and nephelite. 
Miscellaneous conclusions—The preceding discussions emphasize 
the possibility of the formation of widely diverse modes from chem- 
ically identical magmas, these divergencies of the mode from the norm 
or from each other taking place in different directions and to vary- 
ing degrees, from practically complete agreement with the norm to 
the greatest possible divergence from it, though these last are less 
