MARK IN CALCAREOUS SEDIMENTS 



141 



temperature tends to produce more widely spaced mud cracks than 

 a lower one, according to the writer's experiments.^ 



With reference to curvature or flatness of the surfaces of the 

 polygons, the behavior was nearly but not entirely uniform. With 

 a very few exceptions, the polygons showed flat or very nearly flat 



Fig. 2. — Mud cracks with flat- topped polygons i' to 35' in diameter; Miami, Florida 



Fig. 3. — Calcareous mud cracks splitting into laminae; Miami, Florida 



surfaces (Figs. 1-3). In the case of an area of small polygons, a 

 few examples of polygons with upturned margins were noted (Fig. 4) . 

 In some instances the reverse curvature was noted, the margins of 

 the polygons curving down ^ inch or a little more. 



The observations made at Miami and Key West appear to 

 indicate that absolute uniformity does not prevail in the behavior 

 of saline mud cracks with reference to curling. In the great 



^ Op. ciL, p. 143. 



