CRYSTALLIZATION IN IGNEOUS ROCKS 463 
in the above statement, but apart from the question of likelihood 
there is the evidence of small deep-seated bodies to be considered. 
These bodies crystallize under conditions comparable with those 
for a granite except that the rate of cooling is greater. When of 
granitic composition these bodies persist in showing phenocrysts 
of alkali feldspar and often of quartz also (Fig. 5a). 
A study of Fig. 5 shows how such a result is possible. Quartz 
and alkali feldspar begin to crystallize earliest and at first the 
viscosity is low (relatively) so that the crystals formed grow to a 
considerable size. At some temperature (¢,) viscosity has increased 
to such a value that crystallization of quartz and alkali feldspar 
must proceed in the quickly cooled body by the formation of new 
centers. The result is a rock with phenocrysts of quartz and alkali 
feldspar and a matrix showing a second “‘generation”’ of quartz 
and alkali feldspar together with the other minerals. In the case 
of the granite crystallization proceeds in a similar manner and at 
the temperature (¢,) viscosity has reached a comparable value but 
the progress of crystallization is much slower and continues to 
take place by addition’ to the early quartz and alkali-feldspar 
crystals. Lime-alkali feldspar, ferromagnesian minerals, and 
accessories begin and cease to crystallize as the diagram indicates; 
quartz and alkali-feldspar crystals continue to increase in size. 
Feldspar ceases to grow at #,. At this stage quartz, like the feldspar, 
locally shows idiomorphism against the small amount of remaining 
liquid, but in the further growth of the crystals their outline is 
determined by the space available. The result is that the quartz 
crystals have an interstitial relation and there is evidence of only 
one ‘‘generation”’ of quartz, although the interior parts of the 
crystals formed during the earliest stages of crystallization. 
The diagram affords, therefore, a systematic explanation for 
the textural varieties of the granite-rhyolite group as a result of 
different rates of cooling. The commonly stated “order of crys- 
tallization’”’ is contradictory to the evidence of these textural 
varieties. 
The early beginning of crystallization of quartz and feldspar 
is In no respect contradictory to the relations seen in thin sections 
of granites. It accords with the fact that Lagorio? failed to find 
1A. Harker, op. cit., p. 267. 27T.M.P.M. (2), VIII (1887), 421-520. 
