4i6 PRINCIPLES OF STRATIGRAPHY 



generally with a steep inclination dependent on the slope of the 

 reef itself, but it will form nearly horizontal beds at a distance from 

 the reef. Close to the reef the clastic material is often a rudyte, 

 consisting of worn fragments of coral embedded in the sand. Cal- 

 carenytes are, however, the most characteristic clastic sediments 

 around the reef, while calcilutytes or lime-mud rocks are deposited 

 at a distance, where the fine lime mud settles in deeper and more 

 quiet water. Around modern coral reefs the water after a storm 

 is milky for great distances, owing to the presence in suspension of 

 the lime-rock flour. Agassiz has noticed this fine sediment in sus- 

 pension at a distance of 12 to 20 kilometers from the reef. After 

 a prolonged storm 4 to 5 centimeters of coral mud were laid down 

 between two tides. (Agassiz-2.) 



In the lagoon of Keeling atoll the fine lime debris resulting 

 from the erosion of the reef appears like chalk, but when dry is 

 seen to be very fine sand. Finer grained, purely calcareous lime 

 mud forms large soft banks from the southeast shore of the lagoon, 

 covered with a thick growth of Fucus. Similar material is obtained 

 from the Bermuda reefs and has often been mistaken for true 

 chalk. Dunes of these sands are not infrequently heaped up on 

 the higher parts of the reef, and the steep, submerged slopes are 

 covered with them, even where the angle of the slope is as high as 

 55°. Similar white lime sands and muds are deposited along the 

 Brazilian coast for a space of 1,300 miles, from the Abrolhos Islands 

 to Maranhao. 



The sand and rolled coral heads soon become cemented into a 

 hard platform of coral rock more or less structureless in character, 

 this being accompanied in many cases by the growth of Lithotham- 

 nion. In Keeling atoll this flat submerged platform "varies in 

 width from 100 to 200, or even 300, yards, and is strewed with a 

 few large fragments of coral torn up during gales." (Darwin — 21.) 

 This platform is uncovered only at low water, and Darwin found 

 the rock so well cemented that he could break off a piece only with 

 the aid of a chisel. Calcarenytes composed of the rounded par- 

 ticles of shells, corals, spines of echini, and other calcareous struc- 

 tures occur with the rocks of pure coral mud. These sands are 

 found even where the slopes are as high as 50°. The reef rock 

 itself is often so altered in its older portion that no organic struc- 

 ture can be ascertained in it. 



Close to the reef there is often an interlocking of organic and 

 clastic lime rock, for at intervals the reef organisms will extend 

 outward over the region previously covered by the coral sand, after 

 which this sand may again overwhelm the fringe of the reef. 



