128 THE TORTUGAS AND FLORIDA REEFS. 



coarser or finer, according to the exposure of the beach, and the finest sand is found 

 in the most sheltered places, where the silt has free chance to settle. (See Plate IX. 

 for a view of a characteristic coral sand-beach at Key West.) The scarcity of fossils 

 in the coral limestones of the reef has already been dwelt upon, and their absence 

 is readily accounted for from the constant disturbance of the shore-line deposits, re- 

 ducing little by little the larger fragments of shells and corals, or Echinoderms, to a 

 breccia, or again to oolite or fine sand. This is nowhere as well seen as on the shore 

 line of Key West to the north of Fort Taylor (Plate XII.). There the outer reef is 

 sufficiently distant to allow waves of considerable size to break upon this coast, and 

 then strike upon a low line of shore rocks. These rocks are completely riddled by 

 larger or smaller cavities made by boring Mollusks, Annelids, Sea-urchins, etc., or left 

 by fossils or fragments of corals which have fallen out. Thus weakened, large masses 

 are easily undermined by the water, which washes around them with considerable 

 force. They fall off, become then broken again into smaller and finer pieces, which 

 are again reground in their turn, and are finally either resold ered into finer breccia 

 or coarse oolite, according to the composition of the rock, or into the finest oolite or 

 sand. This is then cemented again to the shore line, forming a new line, more or less 

 regularly stratified, dipping towards the sea, and which, when exposed to the action 

 of the air, soon becomes coated with a thin film of hard limestone. This hardens, and 

 forms the ringing crust of the rocks found everywhere on the keys. This coating is 

 formed with great rapidity, an exposure between two tides is sufficient to form such 

 a thin coating, as I have repeatedly had occasion to observe in the deposition of finer 

 oolitic sands which fill the rock pockets just within reach of the waves at high tide. 

 A process of undermining similar to what has been observed at Key West takes place 

 along all the coral rock shores which happen to be exposed to the action of the sea. 

 From the description of Rein and others, this undermining action, acting on a very 

 much larger scale on aeolian deposits of considerable altitudes, must be the principal 

 agent in the formation of some of the peculiarly characteristic features of the Bermuda 



1 -lands. On the east and west shore of Loggerhead, near the northern extremity, we 



can trace admirably the successive layers of the coral limestone which have been 

 deposited and have had an opportunity to harden between the tides, forming what 

 appear to be stratified beds, with their outcrops running as a general thing parallel 

 to the bend of the shore or at a slight angle from it, and dipping on the one side to 

 the eastward and on the other to the westward. 



The bank to the west of the Tortugas has large heads of Astrseans and Madre- 

 pores growing up in it at a depth of from six to seven fathoms. Gorgoniae are 



