FORMATION OF SALT CRYSTALS 



477 



diately sinks. A slight jarring of the water when these crystals are form- 

 ing results in many of the smaller ones showering to the bottom. Crys- 

 tals of this type after sinking to the bottom perfect the cubical shape by 

 growing a thin layer of salt over the base of the inverted pyramid, thus 

 leaving a hollow pyramid inside the cube of salt in which water is often 

 inclosed. Salt crystals formed at temperatures about 110 degrees are apt 

 to be larger than those formed at lower temperatures. They usually de- 

 velop at the surface of the brine from a very small initial cube of salt, 

 which is partially submerged by the growth from its four upper angles 

 of four other small cubes, to each of which still others are added. A thin 



Figure 3. — Pseudomorphs of Salt Crystals 



The crystals occur with hopper-shaped faces on huff-colored Cambrian dolomite from 

 Roche Miette, Jasper Park, British Columbia. X % 



walled, transversely striated, inverted pyramid results, whose angles are 

 formed of miniature superposed cubes. 



An interesting feature of salt crystallization is the control exercised 

 over the crystal form by the temperature at which evaporation and crystal 

 growth occurs. Salt crystals formed under a temperature of 70 degrees 

 Fahrenheit or lower were all of cubical, tabular, or prismatic form in my 

 experiments. When the crystals are formed by evaporation at higher 

 temperatures instead of the cubical and prismatic forms, hopper-shaped 

 crystals are produced. Most of the salt crystal pseudomorphs in sedi- 

 mentary rocks which have come under my notice are of the pyramidal or 

 hopper-faced cube type, like those shown in figure 3, and hence appear to 

 furnish evidence of their formation under the influence of warm climatic 



