Chap, i.] SAND DUNES. I7 



siderable number of Foraminiferous shells occur in the sand, 

 and no doubt careful examination would reveal the presence 

 of fragments of tubes of Serpii/ce, corals, calcareous algae, 

 Bryozoa, and Cirrhipede shells ; but there can be no doubt 

 that by far the greater mass is derived from the shells of 

 Mollusca.-* Thus, although the foundations of Bermuda, and 

 its natural breakwaters and protections, without which it would 

 not exist, are formed by corals, the part above water is mostly 

 derived from another source, and even below the water the 

 same is the case for some distance, for the same beds of sand- 

 stone were met with in an excavation carried to a depth of 

 50 feet. 



The shells, more or less broken, are thrown up upon the 

 beach, and there pounded by the surf. As the tide recedes, 

 the resulting calcareous sand is rapidly dried by the sun, and 

 the finer particles are borne off inland by the wind, to be 

 heaped up into the dome-shaped dunes. The rain, charged 

 with carbonic acid, percolates through the dunes, and taking 

 lime into solution, re-deposits it as a cement, binding the sand 

 grains together.t Successive showers of rain, occurring at 

 irregular intervals, some charged more, some less highly, with 

 carbonic acid, and forming each a crust on the surface of the 

 dune of varying thickness, produce a series of very thin, hard 

 layers in the mass of sand, alternating with seams of less 

 consolidated and sometimes quite loose sand. Crusts of 

 consolidated sand are to be observed commonly on the sur- 

 faces of fresh sand dunes. These layers or strata of the 

 hardened sand follow in form the contour of the dunes, and 

 thus, where these have been perfect domes or mounds, dip 

 outwards in all directions, with curved surfaces from a cen- 

 tral vertical axis. Such an arrangement is constantly to be 

 seen where sections of the older rocks are exposed. I saw 

 especially good instances of it in a small island, near Castle 

 Island in Harrington Sound. Where banks or long rounded 

 ridges of sand have been formed, strata following the surfaces 

 of these in inclination are produced. 



All kinds of curious irregularities in arrangement are to be 



' found in the bedding of the strata, resulting evidently from the 



encroachment of one dune upon the edge of another, or the 



* It would be of great interest to determine, by careful microscopic 

 examination, what are the relative percentages of the very various cal- 

 careous structures composing the calcareous sands of coral islands in 

 different parts of the world. I collected specimens of all the calcareous 

 sands accessible during the voyage of the "Challenger" with that object. 

 They vary very much in composition, some being mainly Foraminiferous. 



t The process is described by Jukes in his account of Raines Islet. 

 " Voyage of the ' Fly,' " p. 339, and elsewhere. 



