248 HAROLD L. ALLING 
follow from this phenomenon are that, in the simple freezing ot a 
magma, the maximum number of stable feldspathic phases is 
two, and these possess fairly constant compositions. The feld- 
spathic content of igneous rocks, therefore, is comparatively simple 
in contrast with its complexity generally found in sedimentary 
rocks. This fact constitutes a useful criterion in distinguishing 
orthogneisses from paragneisses. 
The complete thermo-equilibrium space model of these feld- 
spars should show the dimorphism of the potash and the soda 
components. The discussion of this phase of the subject has 
shown that we do not possess sufficient data to supply the lines, 
the surfaces, and the spaces within the model to make it complete. 
The lack of this information seriously, handicaps the interpretative 
petrologist. It is hoped, in spite of the obvious difficulties, that 
laboratory experimental work may supply the missing infor- 
mation. 
Physical Properties of the Potash-Soda-Lime Feldspars.—In deal- 
ing with a two-component system the change in the physical 
properties and their relations to the change in composition is best 
indicated by lines upon a plane surface. In the case of three com- 
ponents, as here under consideration, these variables are repre- 
sented by surfaces. The most convenient method of indicating 
to the eye the nature of these surfaces is by the use of contours. 
On these the liquidus and the solidus surfaces may be shown by 
lines of equal temperature or isotherms, the value of the extinction 
angles by isogonic lines, and the values of the specific gravity by 
lines indicating equal density, etc. All of these contoured surfaces 
are drawn upon triangular bases showing the interrelationships of 
these properties to the composition itself. To be able to show 
with accuracy properties of the potash-soda-lime feldspars by 
means of these surfaces is an ideal not yet fully realized. If the 
reader will bear in mind that the following diagrams are conjectural 
because of our total lack of definite information about the potash- 
lime feldspars and to be studied as the stratigrapher studies and 
interprets his paleogeographic maps then there will be no mis- 
understanding in regard to them. ‘The writer feels that there is 
great value in the construction of these tentative diagrams, for 
