FORMATION OF LEUCITE IN IGNEOUS ROCKS 359 
the norm, and a square that leucite is not present in the norm. A 
solid color denotes that leucite is present in the mode, and a simple 
outline with uncolored interior that leucite is not present in the mode. 
Thus a solidly colored circle shows that the rock contains leucite 
both in the norm and in the mode; a solidly colored square, that it 
contains no normative leucite, but that this mineral occurs modally; 
an uncolored circle, that, while the norm of the rock shows leucite, 
none of this has actually developed; and finally, an uncolored square, 
that the rock is not leucitic, either normatively or modally. 
The first general survey of Plate II shows that the general position 
and drift of the leucite rocks correspond well with the plotted areas 
of leucitic norms, Lt'O"N‘ to L9O°N°2. A very few fall outside and 
to the left of these limits, these rocks being without normative leucite, 
and some to the right of the area L°O°N°, these being all normatively 
leucitic, but modally either leucitic or non-leucitic. The general 
direction of the axis of this area of the loci of leucitic rocks, or its 
drift, as it may be termed, is distinctly parallel to that of the inclina- 
tion of the leucitic areas, especially to that of the lines OV. 
Again, it will be observed that the abnormatively modal rocks 
(as regards leucite) lie for the most part toward the extreme right and 
left, while those which are normatively modal as regards the presence 
of leucite (represented by colored circles) occupy in general an inter- 
mediate position. Thus the rocks with leucite in the norm but not 
in the mode, and therefore abnormative in this respect, (represented 
by uncolored circles), are found, with few exceptions, well to the 
right or toward the low silica side of the diagram. On the other hand, 
the rocks with leucite in the mode but not in the norm (represented 
by colored squares) are almost all well toward the left or toward 
the side of high silica, and those free from either normative or modal 
leucite still more to the left. 
A further fact which is to be noted is that none of the rocks with 
abnormatively modal leucite fall inside the quartz-orthoclase-albite 
area. OAQ, in this conforming to the conclusion reached above, that 
leucite is not formed from magmas with an excess of silica or norma- 
tive quartz. 
Advancing more into details in our comparison, we must determine 
whether the rocks with leucite in the norm, or in the mode, or in both, 
