S. Taker — Growth of Crystals. 551 



It may clo this in two ways : if the pressure exerted by the 

 crystal of A on B renders the latter more soluble, and suffi- 

 cient time is allowed, B will go into solution where the pres- 

 sure is greatest, and crystallize out again where the pressure is 

 least ; but should conditions be otherwise, B must be pushed 

 out of the way by the growing crystal of A. 



Water is universally present in rocks, though often in very 

 small quantities, and the recrystallization of minerals is largely 

 brought about through the agency of aqueous solutions. The 

 growth of garnet crystals in certain quartz-mica schists studied 

 by the writer furnishes a good illustration of the processes 

 described above. The garnets referred to are of later origin 

 than the schistosity of the rock, which, under the microscope, 

 is seen to consist essentially of elongated quartz grains and 

 small flakes of mica, the two minerals being interleaved to 

 form thin layers or imperfect alternating bands. The garnets 

 in their growth have pushed aside the relatively insoluble 

 plates of mica so that opposite the garnets the mica layers run 

 closer together, while the more soluble quartz has been removed 

 from these points of greatest pressure, and redeposited between 

 the folia, where the supporting effect of the garnets resulted in 

 a reduction of pressure. The recrystallized quartz grains are 

 roughly elongated and oriented, so that their longer dimen- 

 sional axes radiate outward from the garnets.* 



So far in this discussion only the growth of individual crys- 

 tals has been considered. In the growth of crystalline 

 aggregates the molecular forces controlling the formation of 

 crystal faces can have little to do with the shaping of the mass 

 as a whole, which is, therefore, determined by the external 

 forces resisting growth and the accessibility of the material 

 from which the growing mass is composed. During the 

 growth of such a mass, the tendency is to produce that form 

 which requires the least expenditure of energy, and if the 

 resistance to growth were equal in all directions, the growing 

 body would tend to become spherical ; but usually the accessi- 

 bility of material is an influential, if not the controlling, factor. 

 The shape of calcareous concretions formed in shales furnishes 

 an illustration of the growth of crystalline masses where growth 

 is opposed by the resistance of external forces. 



The hollow faces and also the needle-like columnar crystals, 

 obtained in the experiments described in this paper', are to be 

 attributed to the limited accessibility of the materials neces- 

 sary for growth. The formation of the hollow faces, as already 

 explained, is due to the rapid growth of the outer edge of a 

 crystal face in contact with a smooth supporting surface, and 



* Taber, S., Geology of the Gold Belt in the James River Basin, Virginia, 

 Va. Geol. Surv., Bull. No. VII, pp. 29-30 and 228-229, 1913. 



