20 



A NATURALIST ON THE "CHALLENGER. 



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

 into the dome-shaped dimes. 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 .* 

 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 com- 

 monly on the surfaces 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 

 action of various eddies of wind, or the burying of a small dune 

 in the edge of a larger one. In some cases, an already hardened 



STKATA OF SAND ROCK, CASTLE HARBOUR, BERMUDA. 



dune, after having suffered denudation by the action of the 

 waves, lias become buried in a more recent sand mound, and 



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

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



