142 



E. M. KINDLE 



majority of cases, however, the surface layers of the polygons either 

 remain fiat or warp down at the margins. The downcurling appears 

 to be general only when the thickness of the mud is very slight as 

 in the cases observed at Key West. 



RIPPLE MARKS 



With few exceptions the published observations on ripple marks 

 of recent deposits relate to quartz sand ripple marks. In subtropical 



Fig. 4. — Calcareous mud cracks with small polygons showing in some cases poly- 

 gons with margins slightly upcuryed; Miami, Florida. 



and equatorial latitudes, calcareous sand frequently entirely sup- 

 plants quartz sand on the beaches and in near shore waters. The 

 specific gravity of the two kinds of sand is essentially the same and 

 they react to wave and current action in much the same way. 



It may be profitable to describe briefly the conditions under 

 which ripple marks are now being impressed on calcareous sediments 

 or potential limestones at certain localities in the Florida-Bahama 

 region easily accessible from main routes of travel. 



The waters adjacent to Providence Island in the Bahamas 

 afford good opportunities to see ripple marks on calcareous sand at 

 certain localities near the city of Nassau. The channel between 



