22 



PHYSICAL GEOGRAPHY. 



[Part I. 



All idea of the general aspect of Cejdon will be formed 

 from what has here been described. Nearly four parts 

 of the island are undulating plains, shghtly diversified 



tides, so will the fresh water near the 

 surface." — Nafuralist''s Journal, ch. 

 XX. But subsequent experiments 

 have demonstrated that the idea of 

 separating the salt hy filtration is not 

 altogether imaginarj'', as Darwin 

 seems to have then supposed, and 

 Mr. WiXT, in a remarkable paper 

 On a peculiar poiver possessed hy 

 Poroics Media of removing matters 

 from solution in water, has since suc- 

 ceeded in showing that " water con- 

 taining considerable quantities of 

 saline matter in solution may, by 

 merely percolating through great 

 masses of porous strata during long- 

 periods, be gi'adually deprived of its 

 salts to such an extent as prohably to 

 render even sea-water fresh^ — Pkilos. 

 Mag., 1856. Divesting the subject 

 therefore of this difficulty, other 

 doubts would appear to suggest them- 

 selves as to the applicability of Dar- 

 win's theory to coral formations in 

 general. For instance, it might be 

 supposed that rain falling on a sub- 

 stance already saturated with mois- 

 ture, would flow oft'instead of sinking 

 into it; and that being of less specific 

 gravity than salt water, it would fail 

 to "displace an equal bidk" of the 

 latter. There are some extraordinaiy 

 but well attested statements of a thin 

 layer of fresh water being foimd on 

 the surface of the sea, after heavj' rains 

 in the Bay of Bengal. (Journ. Asiat. 

 Soc. Beng. vol. v. p. 239.) Besides, 

 I fancy that in the majority of atolls 

 and coral islands the quantity of rain 

 which so small an area is calcvdated 

 to intercept would be insufficient of 

 itself to accoimt for the extraordinaiy 

 abundance of fresh water daily drawn 

 from the wells. For instance, the 

 superficial extent of each of the Lac- 

 cadives is but two or three square 

 miles, the surface soil resting on a 

 crust of coral, beneath which is a 

 stratum of sand ; and yet on reaching 

 the latter, fresh water flows in such 

 profusion, tliat ^^'ells and large tanks 



for soaking coco-nut fibre are formed 

 in any place by merely " breaking 

 through the crust and taking out the 

 sand." — 3Iadras Journal, vol. xiv. 

 It is cm-ious that the abundant sup- 

 ply of water in these wells should 

 have attracted the attention of the 

 early navigators, and Cosmas Lidico- 

 pleustes, writing in the sixth century, 

 speaks of the numerous small islands 

 ofi" the coast of Taprobane, with 

 abundance of fresh water and coco- 

 nut palms, although these islands 

 rest on a bed of sand. (Cosmas Ind. 

 ed, Thevenot, vol. i. p. 3, 20). It is 

 remarkable that in the little island of 

 Ramisseram, one of the chain which 

 connects Adam's Bridge with the In- 

 dian continent, fresh water is fomid 

 freely on sinking for it in the sand. 

 But this is not the case in the adj a- 

 cent island of Manaar, which partici- 

 pates in the geologic character of the 

 interior of Ceylon. The fresh water 

 in the Laccadive wells always fluc- 

 tuates with the rise and fall of the 

 tides. In some rare instances, as on 

 the little island of Bitra, which is the 

 smallest inhabited spot in the group, 

 the water, though abundant, is brack- 

 ish, but this is susceptible of an ex- 

 planation quite consistent with the 

 experiments of Mr. Witt, which 

 require that the pi'ocess of perco- 

 lation shall be continued "during 

 long periods and through great tnasses 

 of j)0}-ous strata;" Darwin equally 

 concedes that to keep the rain fresh 

 when banked in, as he assumes, by 

 the sea, the mass of madrepore must 

 be " sufficiently thick to prevent 

 mechanical admixture ; and where 

 the land consists of loose blocks of 

 coral with open interstices, the water, 

 if a well be dug, is brackish." Con- 

 ditions analogous to all these parti- 

 cularised, present themselves at 

 Jaffna, and seem to indicate that the 

 extent to which fresh water is found 

 there, is directly connected with per- 

 colation from the sea. The quantity 



