CRYSTALLIZATION IN IGNEOUS ROCKS 461 
and to quickly plunge the charge into a cool liquid which chills 
it instantly to a temperature at which further crystallization is 
permanently checked or rendered excessively rapid (metals). 
The examination of the chilled product reveals those constituents 
which had crystallized at the temperature of the furnace and by 
repeating at other temperatures the whole course of crystalliza- 
tion is followed. ‘The method is known as the method of quenching. 
The “order of crystallization”? commonly stated for granite 
is: accessories, ferromagnesian minerals, lime-alkali feldspar, 
alkali feldspar, quartz. This order is determined from the out- 
line relations of the minerals of granites which, as we have seen, 
can give with certainty only the order of cessation of crystalliza- 
tion, but for the moment it will be assumed that the order is as 
stated and the consequences examined. ‘The order may be repre- 
sented diagrammatically as follows. 
If rocks of granitic composition could be found in which crys- 
tallization had proceeded to a certain stage and had then been 
checked, the product should show the 
following characteristics. Accessories : Pp 
in average amount comparable with c 
that in granites should in general be fi 
present; the most common _ pheno- rae > 
euysts, vely olen the only phenocrysts, tn Figs, 4 «, and 6, A indi- 
should be of the ferromagnesian min- cates accessories, P, ferro- 
erals; in rocks in which crystallization sascas Chon ny cos 
had proceeded to a considerable extent .),.); flieoae tnd oO! a net 
lime-alkali feldspar should accompany 
the ferromagnesian minerals; with more complete crystallization 
alkali feldspar should accompany the preceding minerals, and in a 
rock largely crystalline quartz should appear in company with all 
the above minerals. 
Thousands of rocks of granitic composition are known in which 
crystallization has been checked at various stages (rhyolites) but 
they show no such peculiarities as are to be expected from the 
above outline. They often show phenocrysts of quartz and alkali 
feldspar and of these alone with the greater part of the rock still 
1 Cf. Pirsson, Rocks and Rock Minerals, 1909, 148. 
