DESICCATION OF SALINE MUDS 481 



vessels of the same size, the same quantity from the same mixture being 

 used in the parallel tests. The salt which Avas added to one sample in 

 each case was the only factor which was allowed to differ in the two lots. 

 In each case the dry saline mud showed numerous small cavities which 

 were absent in the corresponding sample of fresh-water mud. 



The mud-cracked clay of the salt playas of the Salt Eiver valley shows 

 very clearly the same peculiarity, the sun-baked saline mud being closely 

 pitted with numerous very minute cavities and irregular pipelike passages 

 (figure 4). 



Considerable geologic importance attaches to this feature because it is 

 one which would in many cases almost certainly be preserved perma- 

 nently in the rocks. It should therefore furnish in the case of fine-tex- 

 tured elastics deposited under subaerial conditions decisive evidence as 

 to whether they originated in saline or fresh waters. 



In another experiment, instead of mud a mixture of air-slaked lime 

 and water was used. One quart of powdered air-slaked lime was placed 

 in each of two pans and mixed with about 2 quarts of water and left to 

 desiccate in a temperature ranging from 75 to 90 degrees. A small quan- 

 tity of salt was mixed with the contents of one pan ; in other respects the 

 two mixtures were identical in kind and amounts of constituents. The^ 

 first mud-crack appeared in the fresh-water pan four days after prepar- 

 ing the mixture. This pan was completely mud-cracked four hours after 

 the first crack appeared (see figure 5). 



The salt mixture, though apparently dry on top, showed no trace of 

 mud-crack for nearly two days after the development of mud-crack in the 

 fresh-water pan. On the sixth day, however, the surface of the saline 

 mixture showed weak lines slightly depressed, but not actually cutting the 

 surface, which marked it into numerous small polygons. One week after 

 starting the desiccation these lines had become more sharply defined, but 

 were otherwise changed but slightly. In this more completed definition 

 they formed very shallow V-shaped valley-like lines of depression, which 

 marked the surface, and the surface only, into polygons (see figure 6) — 

 that is, they had no appreciable downward extension like ordinary mud- 

 cracks and the polygons were flat, except for the slight down-beveling of 

 the edges adjacent to the lines separating them. The fresh-water mud- 

 crack did not change after the day it appeared, except that the intervals 

 between the polygons widened slightly. They showed a width of from 

 one-eighth to three-eighths of an inch. The larger polygons developed 

 around the margin an upwarp of about one-fifth of an inch. 



It is apparent from these experiments that the development of mud- 

 cracks in calcareous mixtures proceeds as in ordinary mud, and that lime 



