THE FORMATION OF LEUCITE IN IGNEOUS ROCKS 
Continued 
HENRY S. WASHINGTON 
COMPARISON 
In the preceding discussion we have dealt only with norms or 
ideal rocks, of simple chemical compositions, and with perfectly 
normative modes composed of a few standard minerals alone. In 
these the relations are purely chemical, and the order of affinity for 
silica has been assumed to be invariable. But we have now to con- 
sider igneous rocks as they actually exist in nature. In dealing with 
them the problem becomes much more complex, not only through the 
introduction of a greater number of chemical and mineral constit- 
uents, but on account of the presence of several factors, both intrinsic 
and extrinsic to the magma or the rock, which may greatly modify 
our assumedly invariable orders of affinity. 
We must remember that few of the standard minerals present in 
rocks are ever as simple in composition as they are assumed to be 
in calculating the norm. ‘Thus a purely potassic orthoclase seldom 
occurs, this mineral almost invariably containing varying, though 
often small, amounts of soda and lime. Similarly with albite and 
anorthite, which tend to crystallize together, as well as to contain 
frequently small quantities of the orthoclase molecule. Leucite 
usually carries small amounts of soda and lime, while modal nephelite 
is never a simple, purely sodic, orthosilicate, but invariably contains 
small percentages of potash, giving rise to a more complex formula. 
The development in very many rocks of the alferric augites, horn- 
blendes, micas, and other minerals, with numerous chemical con- 
stituents and highly complex molecules, is also an important fact to 
be taken into consideration. 
Again, the fact that the crystallization of minerals changes the com- 
position of the remaining fluid portions of the magma must not be 
lost sight of, and finally the influence of such factors as the mass 
action of complex mineral molecules, the physical conditions of 
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