FLORIDA REEFS. 29 
tains wliich 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, dm'ing 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 
