WRIGHT AND HOSTETTERI CRYSTAL GROWTH 415 



2. Examination in polarized light of crystals which have grown 

 under load. In the case of a crystal which has grown under 

 load, the original crystal on application of the load was put into 

 a state of strain; on release of the load the crystal resumed its 

 original state of no strain, or isotropism. In case the layers 

 which were deposited on the strained crystal were laid down in any 

 state of strain other than that of the original crystal, then on the 

 release of the load a difference in the state of strain should ex- 

 ist between these layers and the original crystal, since the same 

 load, or release of same, cannot arbitrarily produce different 

 states of strain in the same material under the same conditions. 

 On the other hand if examination shows that there is no differ- 

 ence in strain between the new and the old material after release 

 of the load the conclusion is valid that the material was de- 

 posited in the state of strain of the original crystal at the time of 

 deposition. 



Examination under the petrographic microscope of crystals of 

 alum and of sodium chlorate on which the outer layers had 

 grown while the crystal was under load, showed no difference in 

 state between the outer and the inner parts of the crystals. Most 

 of the crystals were not uniformly isotropic but exhibited areas of 

 compression and of tension (differences in interference color) and 

 these areas passed indiscriminately and without break from the 

 central part to the outer layers of the crystal, thus confirming 

 the evidence gained by direct observation of crystals growing 

 under load. 



Geologic field evidence. To the student of rocks the presence, 

 in metamorphic rocks which have been in part recrystallized, of 

 layers of fresh crystal substance on original crystals which at 

 the time of deposition were under heavy strain is a fact of common 

 observation. Thus in quart zites and metamorphic sandstones 

 the further growth, under load, of the original sand grains by 

 the deposition of fresh material is clearly shown; similarly the 

 growth of feldspars. On the other hand the growth of such 

 metamorphic minerals as garnet, which are commonly isotropic 

 when examined under the microscope but. which at the time of 

 formation were under conditions of heavy strain, proves that the 



