AGASSIZ: BERMUDAS. 259 



an uncommon thing to see on some of the beaches large slabs of base 

 rock, upon the hard ringing surface of which grow Algae ; these collect par- 

 ticles of sand, and thus form a coating from a quarter to three quarters of 

 an inch thick, upon which larger Alga? then flourish. In the intermediate 

 spaces grow the Serpuke, Mytilus, and Corallines, which soon conceal the 

 surface of the ledge by their protecting coat. Should this slab alone or 

 with adjoining slabs form an extensive ledge far enough out from the 

 beach to be exposed to the action of the breakers, its nearly vertical 

 sides would form a rampart over which the sea combs and pounds down 

 over the edge of the slab, striking beyond the outer edge well toward its 

 interior, according to the size of the breaker. If there exist at the 

 points reached by the breakers any weak spot in the protecting crust, or 

 any incipient fracture, or any difference in the hardness of the upper 

 layer, the sea soon makes an inroad upon it. It grinds out the softer 

 interior parts, which are carried off, and thus forms the beginning of a flat 

 shallow saucer-shaped cup on the inner part of the ledge. The outer rim, 

 on the contrary, protected either by a hard crust or by a growth of Algae 

 and of Serpulae, remains intact, and gradually rises higher and higher, 

 partly from the additional growth of the Serpulae and other calcareous or- 

 ganisms, but maiuly by the grinding away of the interior of the ledge to 

 form a basin, which little by little becomes deeper. The organic growth on 

 the outer rim is more vigorous than in the basin itself, either on its sides 

 or on the bottom, where the sea breaks and is at work grinding away the 

 protecting growth. The Algae, Serpulae, and other growths become less 

 abundant in pi-oportion to their distance from the outer weather rim, 

 until, towards the centre of the atoll, the inner ring or cup or slope is cov- 

 ered with sand. We thus have diminutive atolls, barrier reefs, or crescent- 

 shaped or horseshoe-shaped structures formed out of the aeolian rocky 

 ledges. Should the sea face of the slab be harder than the faces of the sides 

 or of the leeward side, some of those strata are soon broken through, 

 and gaps made in the rim, forming a crescent-shaped wall or arc with 

 its greatest height seaward, the wall gradually falling to leeward to the 

 level of the ledge, and the raised edge sloping towards the horns of the 

 crescent on either side. The lee face of the crescent-shaped atoll is in such 

 a case thoroughly scoured by the outward rush of the sea, which, carrying 

 with it a certain quantity of the sand that fills the depression, runs 

 off the lower lee side with considerable velocity. The depth of the atolls 

 varies from a few inches to six feet or more. The lee faces of both horse- 

 shoe-shaped and crescent-shaped atolls are frequently so rapidly removed 

 as to leave nothing but a vertical wall of from one to two feet on the 

 weather face of the ledsfe. 



