42 



(5) The absence of microcline from the very fine-grained 



diorite and its advent and increase in the coarser 

 varieties. 



(6) The existence of the albite syenite which, with its 



unusually high soda cont-ent, its richness in 

 Ti02 and PoO^, and the intensity of its contact 

 effects, is clearly marked out as an end product of 

 differentiation . 



These facts surely point to differentiation of a granitic 

 magma by settling out of crystals as pictured by Bowen, and 

 as observed by him in laboratory experiments. The first 

 shower of crystals to reach the bottom of the reservoir would 

 be those of least solubility, chiefly biotite, and plagioclase 

 as basic as the temperature and the composition of the magma 

 would permit. These crystals, owing to the short distance 

 through which they had to fall, and the consequent small chance 

 of growth by accretion of material, were of fine grain, (^'^) but as 

 precipitation continued and crystals from, higher levels sank 

 the grainsize gradually increased. 



Microcline was the last mineral to start crystallizing, and: 

 when it did commence, possibly owing to slow cooling in the 

 upper parts of the magma, the centres of crystallization were 

 few and the crystals grew relatively large. 



In the upper parts of the magma, owing to the con- 

 centration of mineralizers, the freezing points of the minerals 

 were lowered, and one can conceive of a temperature gradient 

 existing, the temperature being higher at the bottom than at 

 the top, and yet at the same time the bottom portion being- 

 much farther advanced than the top towards complete 

 consolidation. 



The mineralizers exercised a selective solvent power over 

 the albite molecules in the magma, and to a great extent 

 prevented them from being precipitated; consequently the 

 proportion of potash in the rocks was increased, and instead 

 of the usual sympathetic variation of the two alkalies in the 

 variation diagram there is an actual antipathy. With 

 increasing precipitation of minerals, the concentration of 

 mineralizers became greater and greater, and as this con- 

 centration took place towards the top of the magma reservoir, 

 in time there resulted a marked difference in the composition 

 of the residual liquor in different parts of the magma. In 

 the lower parts it was (as regards felspathic constituents) a 

 mixture of albite and microcline molecules, but at the top 

 almost if not quite entirely albitic. 



(34) Cf. Bowen, Amer. Jour. Sci., vol. 39, l6l5, p. 177. 



