WRIGHT AND HOSTETTERI CRYSTAL GROWTH 409 



less direct bearing on the problem. The direct lines of evidence 

 are: 



1. Observations in plane polarized light of the relative states 

 of strain of an isotropic crystal, growing under load in a super- 

 saturated solution, and the layers deposited on it. 



2. Examination under the petrographic microscope of the rel- 

 ative states of strain, after removal of the load, of a crystal and 

 the layers which were deposited on it while it was under load. 



3. Examination under the petrographic microscope of the 

 state of strain of certain minerals in rocks — such as schists, 

 gneisses, quartzites — which geologic field evidence proves were 

 in part crystallized under load. 



Less direct proof is afforded by experimental evidence on the 

 relative rate of growth and the increased solubility of crystals 

 under load; and also by the directive influence of lines of thrust 

 on the direction of maximal rate of crystallization, as proved 

 both by geologic field evidence and by experiment. These will 

 be considered in the order named. 



1. The relative state of strain between a loaded crystal and layers 

 freshly deposited upon it from its supersaturated solution. To the 

 crystallographer the process of crystallization of a substance con- 

 sists essentially in the regular orientation, as a result of certain 

 directive interatomic forces, of all molecules of the liquid which 

 come within the range of influence of these forces. The crystal 

 itself consists of regularly oriented series of the component atoms 

 arranged in interpenetrating space lattices. As a result of the 

 mutual interaction of these atoms or groups of atoms around 

 points regularly arranged in space, the properties of the crystal 

 are different in different directions; certain symmetry relations 

 hold and find expression not only in the crystal form but also in 

 the behavior of the crystal toward external forces such as me- 

 chanical forces (pressure, tension), chemical forces (rate of solu- 

 tion, crystal habit, etch figures), vibratory movements set up 

 in the ether (X-rays, light, heat, and electric waves), magnetic 

 forces, etc. The law of force between the atoms has not yet 

 been established; it is known, however, that at relatively short 

 distances (measured in millimicrons) the interatomic attraction 



