LITHOLOGICAL APPLICATIONS. 69 



content per gr. mol. of solid and liquid phase was the same for all com- 

 positions, i. e., that all mixtures and the components separately should 

 have the same melting point a case which is known (Roozeboom, 

 d- and I- camphor oxime) , but is certainly confined to optical antipodes. 

 Another reason for supposing the case to be much less simple than 

 a mere linear relation with equilibrium between solid and liquid phases 

 of identical composition appears at once from a direct application of 

 the phase rule. A necessary condition for equilibrium in any mixture 

 is that the number of phases exceed the number of components by 

 two. If the solid and liquid phases are homogeneous, the number of 

 phases (counting vapor) is only three, and equilibrium can not obtain 

 there. 



LITHOLOGICAL APPLICATIONS. 



Supposing the case for the feldspars to be established, by this line 

 of reasoning, as falling under Type I of Roozeboom's classification, 

 important light is thrown on the significance of zonal structure in 

 feldspars and also on the meaning of its absence. A very considera- 

 ble proportion of the feldspars found in thin sections of rocks show 

 zonal structure, though it is more frequent in effusive lavas than in 

 the granular massive rocks. 



Furthermore, with rare exceptions, the outer zones are more sodic 

 than those which they inclose. The width and definition of the zones 

 vary greatly; they are sometimes sharply separated; not infre- 

 quently they show transitions at the edges of the zones, and occasion- 

 ally the gradation is a continuous one, so that the extinction during 

 a rotation of the slide resembles a shadow moving at a uniform rate. 

 This last case is immediately explicable by Roozeboom's theory. If 

 a feldspar magma of any particular composition were to solidify with- 

 out undercooling, the composition would change continuously during 

 solidification in a perfectly definite manner, within limited ranges of 

 temperature and composition, as has been indicated in the discussion 

 of the theory above, the center being always more calcic than the 

 periphery. 



Homogeneous crystals are also readily explained. If undercooling 

 occurs the magma does not begin to crystallize until it has passed 

 below the range of temperature at which the change in concentra- 

 tion can take place. 



Sharply emphasized zones, or zones showing transitions only at 

 their edges, point to changes in physical conditions during crystalliza- 

 tion. Now abrupt changes of pressure are not likely to be frequent 

 excepting during the act of intrusion or extrusion, but in complex 



