FORMATION OP SALT CRYSTALS 479 



conditions. The subject of the control of crystal form and size by tem- 

 perature is, however, one which demands a more exhaustive and detailed 

 series of experiments than those here discussed before any positive geo- 

 logical deductions are warranted. It is a subject which invites further 

 investigation. 



Desiccation of saline Muds 



In an experiment undertaken for the purpose of observing the separa- 

 tion of salt from mud during slow desiccation the fine textured blue 

 Pleistocene clay of the Ottawa Valley was used. A quantity of finely 

 divided clay was added to a saturated solution of salt which was placed 

 in a small aquarium bowl. The bowl was used rather than a shallow pan 

 in order to give a much longer time for evaporation and separation of the 

 salt. A temperature between 85 and 100 degrees was maintained until 

 the water had evaporated down to the surface of the semi-fluid mud. The 

 desiccation of this material yielded a layer of dry mud three-fourths of 

 an inch thick, covered by about one-half an inch of salt. The complete 

 drying of the mud, even with the aid of the electric fan for a portion of 

 the time, required more than two months — a period more than three 

 times as long as a fresh-water mixture would have required. The sepa- 

 ration of salt from mud during its desiccation resulted in three some- 

 what different varieties of salt. The topmost layer was a pure white in- 

 crustation of salt which first covered the surface of the mud. This con- 

 sisted of minute crystals covering the surface with an irregular frostlike 

 effect. Immediately below this another salt layer formed, which consisted 

 of slender acicular vertical crystals holding a small amount of argillaceous 

 matter in the lower part, which gave them a dull gray color near the mud. 

 These acicular salt crystals also filled some of the first mud-cracks which 

 formed. These mud-cracks which were lined with the thin salt plates and 

 needles would if filled with fine sand or mud have duplicated in appear- 

 ance the unusual looking mud-cracks which were called Dictuolites hechii 

 by Conrad and Hall.^^ There can be little doubt that these peculiar look- 

 ing structures which were supposed to be of organic origin represent mud- 

 cracks which were formed in saline muds. The third form in which the 

 salt separated from this mixture gave rise to cubical crystals with hopper- 

 shaped faces, similar to those of the pseudomorphs shown in figure 3. 

 These were scattered sparingly through the dry mud and occurred in the 

 smaller mud-cracks. These crystals in the desiccated mud had a diameter 

 usually between 2 and 4 millimet'ers and frequently produced cracks in 

 the mud radiating from the angles of the cube. 



Paleontology of New York, vol. ii, 1852, pi. 3, fig. 1. 



