MARK IN CALCAREOUS SEDIMENTS 139 



between saline and non-saline mud cracks are supported by sea- 

 shore observations made in Florida. 



In southern Florida, the calcareous deposits commonly met 

 with on or near the beach consist either of calcareous sand or 

 flocculent calcareous ooze. The two kinds of deposits are apt to 

 be sorted by wave and current action both on the beach and in the 

 shallow waters near shore in much the same manner as ordinary 

 mud and quartz sand, resulting in beaches of pure white or cream- 

 colored sand separated by bays of soft gray ooze containing more or 

 less intermixed calcareous sand. The calcareous sand suffers no 

 appreciable loss of bulk on drying and consequently does not sun- 

 crack. Since this material forms the bathing beaches which are 

 familiar to tourists from the North, the casual visitor is apt to get 

 the impression that mud cracks do not develop on the recently 

 formed calcareous marine deposits. The soft, flocculent, calcareous 

 mud, however, reacts to sun and atmospheric exposure in the same 

 way as ordinary mud. The writer was able to see examples of mud 

 cracks on the calcareous mud at only a few localities. One of these 

 is the shore line on Key West Island between the bathing-beach and 

 the United States Bureau of Fisheries laboratory. The flat lime- 

 stone shore here lies but slightly above the high tide line. At a few 

 points along this shore a thin veneer of calcareous mud occurs both 

 within the intertidal zone and slightly above it, where it has been 

 left during spring tides or periods of unusual inundations accom- 

 panying storms. Along this stretch of shore good examples of mud 

 cracks were seen both in the salt-water zone and above it. In the 

 latter, all the examples observed showed polygons with the margins 

 turning up. In the salt-water mud cracks the margins curved down- 

 ward often sufficiently to lift the center of the polygon clear of the 

 ground. 



Various other stretches of the shore line, both in south Florida 

 and on Providence Island, Bahamas, were examined for mud cracks, 

 but no very satisfactory localities were seen for extensive subaerial 

 exposures on natural beaches of the type of calcareous sediment 

 capable of developing mud cracks. Fortunately, however, the 

 extensive dredging operations, which were undertaken for the 

 purpose of reclaiming swamp land near Miami beach, gave the 



