WAEKEN. — ALKALI-GRANITES AND PORPHYRIES. 309 



minerals to each other in the granite, as revealed by the microscope, 

 does not in itself lead to any very precise conchision regarding the 

 order of crystallization.®^ It might be inferred ®® that the feldspar was 

 perhaps first to crystallize, that the quartz and aegirite and perhaps a 

 part of the feldspar (albite) was the last to cease crystallization. 

 Furthermore, the great complexity of the chemical system represented 

 by the granite, its many components and the several complex, iso- 

 morphous relationships known to exist ])etween some of them, would 

 render any predictions as to the probable order of crystallization of 

 very difficult, even if we had anything like a full knowledge of the 

 thermal and other data bearing on the question. Owing to the 

 differences in the chemical composition of the coarse-granite and the 

 granite-porphyries etc., and the resultant differences in mineral com- 

 positions, we cannot look upon the porphyry as the exact quenched- 

 quickly cooled- equivalent of the granite. However, the figures given 

 show that the differences in mineral composition which would be 

 effective in modifying the order of crystallization are those relating 

 largely to the quartz and feldspar, so that with this in mind we shall 

 find that a comparison of the crystallization in the porphyries with 

 that in the granite leads to a better understanding of the crystallizing 

 process as a whole. 



The phenocrysts of the quickly cooled porphyry represent the first 

 crystallizations from the magma. These appear to be quartz and 

 the homogeneous soda-potash-feldspar crystals. If we should assume 

 that the proportions of quartz to feldspar in Vogt's quartz-feldspar 

 eutectic are approximately correct, then the rather close approach 

 in the amounts of these minerals to this proportion might lead us to 

 expect a simultaneous crystallization. The greater proportion of the 

 quartz phenocrysts to those of feldspar in the contact phases of the 

 granite-porphyry found at the higher contact levels, as shown by the 

 Rosival estimates given in columns II and III, p. 243, and the greater 

 resorption of the quartz phenocrysts are probably indicative that the 

 quartz preceded the feldspar in the quickly chilled phases. On the 

 other hand, at deeper levels of the contact, the proportion of the 

 quartz to feldspar phenocrysts is reversed and the amount of the former 

 is relatively small (see column IV, p. 243). Further, if we consider 

 the rhombenporphyry and assume that, in general, the marginal 



65 In this connection see an interesting paper by N. L. Bowen, Journal of 

 Geology, 20, No. 5 (July-Aug., 1912), dealing with the order of crystallization 

 in rocks. 



66 See Warren & Palaehe, op. cit., p. 143-4; also Murgoci, op. cit., p. 137. 



