AGASSIZ: BERMUDAS. 261 



shore ledges. The inner slope is covered by a thin growth of Corallines 

 and smaller Alga?, which do not seem to thrive as well as where 

 they receive the direct force of the breakers. Close to the westward 

 of this diminutive barrier reef are ledges which may be regarded as 

 typical of the changes which have taken place in a bare aeolian ledge 

 just dropped from the shore cliffs until it becomes a typical boiler of 

 the south shore. One of these is separated from the western edge 

 of the barrier reef by not more than six feet, and has a depth of water 

 of more than ten feet in the passage between them. To one side of this 

 dumbbell-shaped atoll, which is nearly thirteen feet on its longest axis, 

 is another atoll on a mushroom-shaped ledge seven feet in diameter, with 

 a regular rim and a pot-hole of three feet in depth. This is separated 

 from the adjoining mushroom-shaped ledge by a gap of four feet, with a 

 greatest depth of ten feet, and eight feet on the shore edge ; it is sep- 

 arated from the shore and a large ledge to the south by a deep passage 

 of nine feet in width. The length of the larger ledge, irregularly shaped, 

 is over forty feet, and its width varies from twenty to thirty-five feet. 

 Its outline is formed by curved walls, the edges of irregularly shaped 

 elongated or circular dumbbell-shaped pits gouged out from a broad 

 platform of seolian rock. On the lee part of the ledge an irregular 

 rectangular pot-hole has been formed, five feet in depth on one side 

 and four on the other, sloping upward toward the broad outer rim. 

 Comparatively slight variations would change the surface of the led^e 

 into an atoll with a narrow rim following the outlines of the ledge; or 

 it may become divided into two irregularly shaped pits by the coales- 

 cence of the few smaller pits now upon the platform, and a single curved 

 wall, the remnant of the face of one of the circular pits, would form a 

 division wall ; or the sea may break through to a greater extent than it 

 has done already and leave on this ledge only disconnected fragments of 

 wall of varying shape, in which it would be difficult to recognize the 

 walls once limiting circular or dumbbell-shaped pits. Outside of the 

 larger ledge is a pear-shaped atoll with a broad sea face or rim sloping 

 inward, and a circular pot-hole about three feet in depth, the rim of 

 which is narrower to the leeward. 



All these ledges are deeply undercut and abraded, and are mushroom- 

 shaped ; their faces are vertical or nearly so, and all show traces of the 

 action of the sea upon the pillar forming the base. The ledges I have 

 examined close to the shore, half-way to the reef, or on the outer line of 

 ledges, all present modifications of the ledges described. Their ultimate 

 shape depends upon many local factors, and they show but a small num- 



