8. Taber — Growth of Crystals. 553 



factor. Where the- elongated form develops chiefly as a result 

 of external pressure, the longer dimensional axes are normal 

 to the direction of greatest pressure and parallel to the least 

 pressure. Van Hise and others have explained the develop- 

 ment of elongated crystals during the recrystallization of rocks 

 on the theory that solution takes place along the line of 

 greatest strain and deposition along the line of least resistance 

 and normal to the maximum stress. This theory is confirmed 

 by the experiments of F. E. Wright in the formation of crystals 

 of wollastonite, diopsicle and anorthite under vertical pressure.* 

 If the normal habit of a mineral is columnar, then those 

 crystals that are oriented with their longer axes parallel to the 

 least pressure will tend to grow at the expense of those that 

 are not so favorably oriented. When columnar crystals develop 

 parallel to the direction of the greatest pressure, as in the 

 experiments with copper sulphate and ice described in this 

 paper, the orientation is in spite of the pressure and not because 

 of it. 



Attention has been called to the fact that crystallization does 

 not take place so readily in very small capillary spaces. It 

 is possible that the high surface tension and extremely small 

 volume of the solution* may tend to prevent the development 

 of centers of crystallization. 



The solution, furnishing the material necessary for growth 

 to a crystal surface that is under pressure, may consist of a 

 layer so thin that the space occupied must be classed as sub- 

 capillary rather than capillary in size. A subcapillary opening, 

 as defined by Van Hise, is one in which the attraction of the 

 solid molecules extends from wall to wall.f The flow of 

 solutions through such extremely minute openings must be 

 exceedingly slow, and therefore it is probable that the material 

 for growth reaches the growing surface largely as a result of 

 diffusion even in the case of natural crystals which have grown 

 gradually through a long period of time. It would be very 

 difficult, if not impossible, to expel solutions from subcapillary 

 openings solely through the application of pressure, and there- 

 fore, under favorable conditions, the material for growth may 

 be able to reach a crystal that is under very great pressure — a 

 pressure that is perhaps several times the crushing strength 

 of the crystal. 



The pressure that may sometimes be observed during the 

 growth of a crystal is probably due to the molecular forces 

 associated with the separation of solids from solution and the 



* Wright, F. E., Scliistosity byCrystallization, A Quantitative Proof: this 

 Journal, vol. xxii, pp. 224-230, 1906. 



f Van Hise, C. E., Treatise on Metamorphism, U. S. Geol. Snrv., Monograph 

 4?, p. 185, 1904. 



Am. Jour. Sci. — Fourth Series, Vol. XLT, No. 246. —June, 1916. 



