THE MINERALOGRAPHY OF THE FELDSPARS 215 
of Allen and Day. Figure 2 is the equilibrium diagram? of the 
soda-lime series, based upon a molecular percentage instead of a 
weight percentage as has been done in all the other diagrams. 
The study of the solid-solution type of diagram shows us that 
the resulting crystals freezing from a mutual solution will not be 
homogeneous in composition, unless simultaneous or subsequent 
adjustment takes place, and consequently will vary in their optical 
properties as may be seen in zonal-grown crystals. But in deep- 
seated igneous rocks where the time of cooling is long, the non- 
homogeneous crystals gradually become uniform in composition by 
readjustment or exchange with each other and with the surrounding 
liquid. This process occurring between crystals is known as 
diffusion. All gradations between beautifully zoned plagioclase 
crystals and perfectly homogeneous ones occur in nature. The 
degree of homogeneity is therefore a function of the rate of chill, 
or, as the metallurgists would say, a measure of the rapidity of the 
quenching of the silicate alloy. Some zonal textures are, however, 
due to more complex processes such as the reabsorption of the 
margins of the already solid crystals and the adding of a new coat- 
ing or layer deposited thereon. ‘The writer believes that undue 
emphasis has been paid to the latter explanation of such textures 
and entertains the view that a large proportion of zonal crystals 
are not due to “‘magmatic corrosion,” as it is called, but to normal 
magmatic crystallization under the influence of rapid chill. 
Sometimes the crystal zones are sharply defined, each possessing 
fairly constant physical and chemical properties. ‘These can be 
explained by irregularities in the rate of cooling. The reversal 
of the order of zoning may be due to undercooling or the exposure 
of crystals to a liquid of a composition different from that of the 
one from which they crystallized. 
The Properties of the Soda-Lime Feldspars.—It would seem as 
though our present knowledge of the physical properties of the 
plagioclase series was as complete as could be desired. Through 
tA. L. Day and E. T. Allen, “The Isomorphism and Thermal Properties of the 
[Plagioclase] Feldspars,” Carnegie Inst. Pub. 31 (1905). 
’ 2N. L. Bowen, ‘“‘ Melting Phenomena of the Plagioclase Feldspars,” Amer. Jour. 
Sci. (4), XXXV (1913), 583. 
3.N. L. Bowen, Amer. Jour. Sci. (4), XXXV (1913), 507. 
