FLORIDA REEFS. 29 



tains which Gressly has called " Terrains de charriage." They show signs 

 of attrition, but at the same time are so incrusted as to be protected from 

 farther disintegration, during the transportation of the mud in which they 

 are embedded. The general aspect of the reef indicates that all this mud 

 is slowly moving from east to west, and encroaching upon the free growth 

 of the corals ; while it furnishes at the same time the minute materials, 

 which in connection with coral sand fill the intervals between the coral 

 heads and coral boulders. Where it is accumulated by eddies so as to 

 approach the surface, it forms mud flats or may have given rise to the 

 layers of muddy limestone described above. 



Several of the keys adjoining the main range, and standing out somewhat 

 into the channel, differ in structure greatly from the main keys. They are 

 not formed by the accumulation of coral boulders and coral fragments upon 

 the edge of the reef, but by the accumulation of coral sand and mud 

 in eddies or shoals. They are, in fact, the highest mud flats, to the consoli- 

 dation of which the mangrove growth upon them has contributed. Such 

 are Rodriguez Key, Tavernier Key, Sister Key, Loggerhead Key, The 

 formation of similar keys in the course of time may be foretold, off Old 

 Rhodes, upon the Washerwoman's Shoal, and upon the middle ground off 

 Sand Key. Let us suppose the accumulation of coral sand and mud 

 between the main range of keys and the outer reef to have increased so as 

 almost to fill the ship-channel, reducing its deepest part to some eight or 

 nine feet of water, and a number of mangrove keys like Rodriguez and 

 Tavernier to have been formed where we now have only sand shoals or 

 banks and coral heads, as between Carysfort and Key Largo, for instance ; 

 we should then have precisely the same conditions as are now presented by 

 the extensive mud flats lying between the main-land and the keys from 

 Cape Sable to Cape Florida. Although we infer by analogy that, when the 

 main keys were only a reef, their inner side supported live coral heads and 

 was bordered by larger or smaller shoals of coral, and inhabited by a fauna 

 similar to that now found in the ship-channel, nothing of all this is now to 

 be seen in that locality. The coral heads and sand ridges have given place 

 to extensive mud flats, intersected by shallow channels and shallow depres- 

 sions, interspread with innumerable low mangrove islands arranged in little 

 groups, or forming more continuous chains such as the Walker Keys, which 

 stretch almost uninterruptedly, in a southerly direction, from the main- 

 land to Key Largo. In the northernmost part of the reef west of the 



