Origin of Pliowcrysts. — Crosby. 309 
sections were observed. The normal downward sequence is 
dioryte, basic granite or granodioryte with basic or dioritic se- 
gregations, passing into more acid and normal granite, in 
which the dark segregations or inclusions gradually die out. 
The normal granite, free from segregations, becomes appre- 
ciably coarser downward, and at last, in the bottom of the can- 
yon, as at the mouth of Minaret creek, it becomes in part 
coarsely porphyritic, with phenocrysts up to two inches long. 
I can not doubt the normal character of this seciuence, or the 
truly abysmal or plutonic origin of the coarse granite por- 
phyry. 
In seeking an explanation of the development of the por- 
phyritic texture in the very heart of a great batholith, it has 
occurred to me that we may, perhaps, apply to a magma the 
well-known principle that, as recently stated by Van Hise,* 
under a long- continued condition of saturation in a solution 
"the larger crystals grow at the expense of the smaller ones, 
and this process goes on more rapidly in proportion as the 
temperature is high and the pressure is great." "The growth 
of the large crystals at the expense of the small ones is due 
to the fact that the smaller crystals are somewhat more solu- 
ble than the larger. The explanation of this change, as given 
by Ostwald, lies in the surface tension which exists on the 
boundary surfaces between solids and liquids, as on those be- 
tween liquids and gases — the so-called free surfaces of liquids. 
This tension acts so that the surfaces in question are reduced 
in size, with the consequent enlargement of individual crystals. 
During the change for a given volume the lessening of the 
total surface of the crystals, and consequently the lessening of 
the surface tension, results from the fact that the surfaces of 
the crystals will be small in proportion as the individuals are 
large." In other words, if we may assume that in the slow 
development of the crystalline granular texture in a plutonic 
magma, the growing crystals of a particular mineral are not of 
uniform size, and this appears to be inevitable, the larger crys- 
tals will grow at the expense of the smaller ones, thus in- 
creasing the disparity between them. 
It is probable that in a magma this princii)lc would, on 
*Bull. Geol. Soc. Amer., vol. 9, p. 274. 
