CunnincHam—Crystallization of Minerals in Igneous Rocks. 399 
rate of yielding, we obtain a single point on a particular isometric 
corresponding to the pressure at which the experiment is conducted. 
But without making at least one other experiment under a different 
external pressure we can get no idea of the slope of this particular 
isometric, and therefore we cannot even guess what the viscosity 
would be under such enormous pressure as those to which minerals 
would be subjected when crystallizing in the depths of the earth. 
In fact we must determine the isometrics for both felspar and 
quartz before we can know which! will deform the other if crystals 
of the two minerals are conceived to be pressed together during the 
consolidation of a rock magma. 
In the second place this idiomorphie “ crystallizing force” is— 
so far as I know—purely hypothetical, or at any rate such a erys- 
tallizing force is of incomparably smaller dimensions than the 
mechanical force necessary to distort a viscous crystal. It is in 
fact opposed to the principle of Professor James Thomson,” ‘ that 
stresses tending to change the form of any crystals in the saturated 
solutions from which they have been crystallized must give them a 
tendency to dissolve away, and to generate, in substitution for themselves, 
other crystals free from the applied stresses or any equivalent stresses.” 
Now the slow rate of cooling, and therefore the enormous 
time at its disposal will make this ‘tendency ” all important 
in the case of a mineral crystallizing in a molten magma. 
In other words, a crystal will grow round any obstacle, no matter 
how trivial, rather than push aside the obstacle in order to develop 
its perfect crystalline form. This is abundantly demonstrated in 
the case of minerals which constantly grow round microscopic 
crystals rather than remove them by overcoming the small surface 
tension and viscous forces which hold the obstacles in position. 
Is it possible then to conceive of a growing felspar deforming a 
quartz crystal? Professor Boys found that the force necessary to 
draw out “ quartz fibres’ at the temperature of the oxy-hydrogen 
blowpipe to be perceptibly greater than the force necessary to do 
the same for any of the silicates, including felspar. Further, if a 
long period of high viscosity during fusion is to be regarded as an 
1 Cf. Boys, Proc. Phys. Soc. 9 (1887-88), p. 13, &e. 
? James Thomson, ‘‘ On Crystallization and Liquefaction, as influenced by Stresses 
tending to change of form in the Crystals,’’ Proc. Roy. Soc., xi. (1861), p. 474. 
3 Proc. Phys. Soc. 9 (1887-88), p. 13, &e. 
SCIENT. PROC. R.D.S., VOL. IX., PART IV. 2F 
