WARREN. — ALKALI-GRANITES AND PORPHYRIES. 327 



generally, so much so, that their original structures are often pro- 

 foundly changed. 



Quartz and a soda-potash feldspar (mixed crystal) appear as the 

 first crystallizations from the porphyry magma, followed closely by 

 the less sodic-pyroxenes. It is thought, however, that the feldspar 

 was the first crystalline phase to form normally, followed quickly by 

 the quartz and pyroxene. These were followed by the other minerals. 

 It is inferred that in the granite the order of crystallization was sub- 

 stantially the same as that of the granite-porphyry. In the porphyries 

 there was a sharply marked pause closing the phenocrystalline stage 

 of growth, due, it is believed, to supersaturation, after which crystalliza- 

 tion was resumed, but about vastly more numerous centers, the various 

 minerals continuing to grow almost or quite simultaneously, in part 

 attaching themselves to the earlier formed crystals, but largely forming 

 a fined grained groundmass, in part with a poikilitic fabric. In the 

 porphyritic phase of the granite, into which the granite-porphyry 

 passes, the pause in the progress of the crystallization was less marked, 

 and in the normal granite it did not occur, owing to the more perfect 

 conditions of equilibrium conditioned by very slow cooling. The 

 later crystallizations added themselves to the crystals already present, 

 and thus obliterated all distinction between phenocrysts and ground- 

 mass. 



It is held that the granite-porphyry represents substantially the 

 composition of the magma as intruded, and that beneath this protect- 

 ing cover the remaining magma was able to differentiate to a small ex- 

 tent forming a more basic phase, the rhombenporphyry, itself becoming 

 in consequence more siliceous. Assuming that by this differentiation 

 the granite moves toward a eutectic composition, which is by no means 

 a necessary pr certain procedure, it is pointed out that the propor- 

 tions of quartz to the feldspars departs quite widely from the granite 

 eutectic proportions as estimated by Vogt. In fact the proportions 

 of these minerals in the granite-porphyry is nearer to Vogt's eutectic 

 ratio than are those of the granite, which appears to be a variation 

 in the wrong direction to agree with his theory. The effect of the 

 presence of the sodic-iron silicates on the composition of a possible 

 eutectic is not yet determinable, and in any case, the composition of 

 the granite eutectic appears to be a very open question. 



It is suggested that the potash and highly sodic-feldspars may first 

 crystallize as homogeneous mixed-crystals (type I, of Rooseboom) 

 and at somewhat lower temperatures pass though a transition point, 

 or interval, becoming then only partially miscible in the solid state, 



