416 WRIGHT AND HOSTETTER: CRYSTAL GROWTH 



crystal state as modified by any external mechanical system of 

 forces is the state into which freshly precipitated material enters 

 when deposited on the crystal to which it becomes affixed. 



Many more examples of a geologic kind of the growth of min- 

 erals under strain might be cited, but the above suffice to estab- 

 lish the fact that, whatever is the nature of the orienting polar 

 crystal forces, when these are modified slightly by external 

 forces, the resultant of the two systems dominates and orients 

 groups of atoms in the act of crystallization, as they become 

 affixed to the strained crystal, in such a way that they conform 

 to the state of deformation of the original material. 



Evidence of growth and solubility rates. In experiments on the 

 rate of growth and the rate of solution of alum crystals which are 

 not uniformly strained, it has been repeatedly observed that the 

 points of greatest strain dissolve more rapidly than areas of the 

 same crystal which are under less strain; also that on crystal 

 growth the areas of less strain tend to grow more rapidly than 

 areas of greater strain. This indicates that the state of strain 

 of a crystal is a factor to be considered in problems of crystal 

 growth. If the freshly deposited layers did not enter into the 

 state of strain of the original crystal but in an isotropic state 

 there should then be no difference in the rate of growth for dif- 

 ferently strained parts of the crystal; similarly the fact that the 

 rate of solubility is affected by mechanical strains proves that 

 the strained crystal is in a state different from that of the un- 

 stressed crystal. 



Evidence of effect of thrust on direction of crystal growth. That 

 the direction of mechanical thrust has a direct influence on the 

 direction and rate of growth of crystals is proved by geologic 

 field evidence and also by laboratory experiments. 21 In the lab- 

 oratory experiments cubes of wollastonite and other silicate 

 glasses were held under load at a temperature at which crystalli- 

 zation took place slowly, but at which the glass was still rela- 

 tively rigid and able to support a load without much flow. 

 Longitudinal sections across the cubes crystallized under these 

 conditions showed that the prismatic directions of the crystals 



21 Wright, F. E. Schistosity by crystallization. Am. Journ. Sci., (6) 190. 



