AGASSIZ: BERMUDAS. -•"< 



in such structures. For alongside the atolls, either rising to the surface, 

 or near to it, or always covered at low water, there are other ledges, of 

 endless differing shapes, which do not attract the eye, but which play as 

 important a part in the economy of the ledges off the south shore as the 

 atoll-shaped structures themselves, and which give us the clue to their 

 formation. 



Everywhere on the shores of the Bermudas where active degradation 

 of the coast is going on we meet with a number of ledges, or pinna- 

 cles, or islets, or mushroom-shaped rocks, which have been fashioned 

 by the sea into a nearly circular or elliptical form. Sometimes a num- 

 ber of these isolated rocks may stand in a row above high-water mark, 

 the stems almost eaten away from the ledge upon which they stand. 

 When the top tumbles over, the support, or a part of it, may remain well 

 above low-water mark. It is upon these ledges of all sizes, from a foot 

 or so in diameter to long elliptical or irregularly shaped masses of fifty to 

 seventy feet in length, or even more, that the sea begins to act, and to 

 shape the serpuline atolls of the south coast, though they are not con- 

 fined to it, as I shall show hereafter. Standing on some parts of Elbow- 

 Beach, one may follow the irregular mushroom-shaped rock ledges stand- 

 ing between high and low water mark to those at and beyond that point 

 into deeper water. We may note the changes which gradually take place 

 as.the protective growth upon these irregular ledges, at first bare, trans- 

 forms them into the atolls, or crescent-shaped or S-shaped structures 

 forming the reef off the south shore. One can watch at low tide and 

 see the breakers combing in over the rim of the little atoll scouring the 

 lagoon, and the superfluous water flowing over its sides. The sea breaks 

 over the edge, carries off such loose fragments as may have been started 

 by the preceding rollers, and scours the inside with the sand it may have 

 brought in, in addition to what it finds inside. 



Some of the crescent-shaped serpuline reefs are formed on ledges bare 

 at low water extending out from shore. They form low vertical walls of 

 from twelve to twenty-four inches in height, running in a series of irreg- 

 ular curves, a kind of festoon as it were, protecting the inner lagoon or 

 lagoons of all sizes and shapes which have been gouged out by the waves. 

 It is not uncommon on the south shore to find fine sand deposited on 

 the flat ledges near low-water mark, and kept in place by the growth 

 of a thin sward of Algae; this, together with the thin crust formed over 

 its surface, hardens the mass, keeps it in place, and enables it to resist 

 the moderate action of the breakers. 



Going westward from Great Turtle Bay to Warwick Bay we find the 

 vot.. xxvi. — \-o. 2. 17 



