544 S. Taber — Growth of Crystals. 



respect to the surfaces that are rendered more soluble because 

 of greater pressure. In order that growth may also take place 

 on the surfaces subjected to the greater pressure, some means 

 must be provided for maintaining the supersaturation of the 

 solution in contact with them. 



In their conclusions concerning the area of contact between 

 a growing crystal and the supporting surface, Becker and Day 

 state that "there is reason to believe that this area changes 

 constantly as the crystals grow, and is less for a smaller load 

 than for a larger one."* When a supersaturated solution is 

 diffusing under the edge of a growing crystal, growth is most 

 rapid on the outermost edge, and the concentration of the 

 solution decreases rapidly with the distance that it has to 

 diffuse under the crystal. As growth takes place and the crys- 

 tal is raised this tends to keep the outer edge, supporting the 

 weight of the crystal, as narrow as possible, but the area of the 

 edge must always be large enough to keep the pressure per 

 unit area less than the crushing strength. Moreover since the 

 solubility increases with the pressure the area of the support- 

 ing surface must always be sufficient to maintain the relative 

 supersaturation of the solution with respect to it and prevent 

 re-solution. This argument leads to the conclusion that the 

 area of support may also be dependent on the relative concen- 

 tration of the solution with which it is in contact. It is difficult 

 to obtain the quantitative data that would confirm this conclu- 

 sion, but observation seems to support the view that the area 

 is smaller when in contact with a highly supersaturated solu- 

 tion than when the solution is almost at the saturation point. 



The molecular forces enabling crystals to grow in directions 

 in which they must overcome external pressure, furnish the 

 most plausible explanation of certain geologic phenomena ; but 

 hollow faces such as those obtained in the foregoing experi- 

 ments are not common in minerals, and their absence has been 

 used as an argument against this hypothesis. In nature, how- 

 ever, crystal growth usually takes place at a much slower rate 

 than is feasible in laboratory experiments, and the rock masses 

 in contact with a growing crystal are more or less permeable 

 to solutions. That solutions are active agents even in relatively 

 impervious rocks, like granite, is indicated by the reactions 

 that take place not only in fractures but also along the contact 

 between different minerals and even along the cleavage planes 

 of the minerals themselves. 



In attempting to reproduce under laboratory control the 

 needle-like ice columns that are commonly found on bare 

 clayey soils after a cool night, the writer observed that water 

 *Loc. cit.. p. 287. 



