142 E. M. KINDLE 



salt along these lines. These small polygons appear to represent 

 the convection cells of Benard, 1 Dauzere, 2 and Sosman, 3 and have 

 no direct relationship to mud-cracks. A few hours after the ap- 

 pearance of the triradiate figures some regular mud-cracks formed 

 in the median portion of the pan, cutting a limited area into rather 

 small polygons. Two of these mud-cracks were extensions of arms 

 of the three-line figures previously mentioned. Two days after the 

 desiccation appeared to have been completed, the remainder of the 

 surface cracked, after being removed from the sun, splitting the 

 entire surface into polygons. A noteworthy feature of these poly- 

 gons is downwarping of their margins and absence of lateral 

 shrinkage, which is in sharp contrast with the upwarping of the 

 sides and considerable shrinkage of polygons which formed from 

 the fresh-water mud. In fact, the 'saline mud showed as a whole 

 slight lateral expansion which was taken up by the arching upward 

 of the median portions of the polygons. 



Considered from the standpoint of preservation as permanent 

 features in the strata of consolidated rocks, mud-cracks in saline 

 clays would have a rather poor chance of permanent preserva- 

 tion owing to their slight breadth. If preserved, they would be 

 quite inconspicuous as compared with ordinary mud-cracks. The 

 star-shaped figures, however, by reason of their broad and deeply 

 incised arms lend themselves well to preservation under natural 

 conditions of sedimentation and should be regarded, when found 

 on rock surfaces, as evidence of subaerial desiccation. This experi- 

 ment represents the behavior of highly saline mud such as would 

 be found on the shores of salt lakes or detached arms of the sea 

 rather than that of the muds ordinarily met with about the estuaries 

 of rivers, which have a much lower degree of salinity. 



The salinity of ordinary estuarine mud was approximated in 

 another experiment. Sea water was used in still another. In all 

 these supplementary experiments, including a sample of mud having 

 less than the salinity of ordinary tide-flat mud, desiccation produced 



1 H. Benard, Les Tourbillons cellulaires dans une nappe liqiiide, etc., thesis, Paris, 

 1901; Rev. gen. Sci., XI (1900), 1261-71, 1309-38. 



2 C. Dauzere, Jour, physique, VI (1907), 892-99; VII (1908), 930-34; Assn. franc, 

 av. sci., 1908, pp. 289-96. 



3 Robert B. Sosman, "Types of Prismatic Structure in Igneous Rocks," Jour. 

 Geol., XXIV (1916), 219-24. 



