NELSON ON THE BAHAMAS. 213 



detached coral blocks, above the tide-line leads us to the supposition 

 that no upheaval has taken place during the present epoch. Nor is 

 any reference to an upheaving agency called for to account for the 

 slight elevation of the dry flat bottoms of the old lagoons, now filled 

 in with rock-marsh *, for the existing natural process of the formation 

 of land in creeks and lagoons by means of mangroves and calciferous 

 confervse, — with or without the preceding assistance of sea-weeds, 

 corallines, deposits of chalk-mud, and sand-drifts, — shows that or- 

 ganic agency, direct or indirect, is quite competent to the task of 

 raising the surface to the height required. 



The fact of detached blocks of coral being found in the rock at 

 considerable distance from the sea-coast at the tide-level, proves that 

 no subsidence has taken place during the present epoch. Conch- 

 shells f also, either dispersed or in beds, are found by the well-diggers 

 in the solid rock at about the sea-level, and thus bear evidence to the 

 same fact. 



8. Ripple-marh and its protection. — Capt. Nelson observes, in 

 Part IV. Section C. of his Memoir, that the extent to which surface- 

 action affects the ground under water is a matter of importance to 

 the Engineer and of interest to the Geologist, not only as regards 

 Ripple-mark, — which may be considered as the record of a self-regis- 

 tering oscillatory motion of a fluid among relatively light pulverulent 

 bodies on which it lies, and which may be seen also on snow and 

 sand|: exposed to any similar reverberatory action of the wind (and 

 indeed the clouds, in the state of "mackerel-sky," may perhaps be 

 said to exhibit an atmospheric ripple-mark), — but also with respect 

 to the preservation of the tracks of Annelids, Molluscs, Crustaceans, 

 Birds, Reptiles, &c. ; and in connection with this subject he quotes 

 the following from his journal: — Gun Cay, December 31, 1850. 

 In 20 feet of water the ripple-marks and the imperfect scorings made 

 by the cable on the white-sand, before it settled down where anchored 

 last night, were woi\derfully perfect this morning ; the water having 

 in nowise destroyed the sharpness of the impressions. The sea is as 

 exquisitely clear at the Bahamas as at the Bermudas ; and, when it 

 has been quite calm, I have seen not only the ripple-mark quite 

 plainly, but the small plants, &c. growing below, and the worm- 

 heaps, at a depth of 11 fathoms ; and the floating corpuscles, — 

 animate or inanimate, — organic or inorganic, — could be seen very 

 distinctly by oblique light, as they emerged from the shadow of the 

 vessel (seen as distinctly on the sand as if it had fallen directly on 

 the land), like dust in a pencil of light traversing a darkened room. 

 Judging from this, and from what I caught in the muslin net this 

 morning, it is probable that the coating of sand in which these ripple- 

 marks, &c. will soon be cased, will be rich in remains of minute 

 organisms, notwithstanding the generally deliquescent character of 

 their substance. 



Conclusion. — The author's observations with regard to the Bahamas 



* See above, p. 203. 



t See above, pp. 20G and 212. 



+ Proc. Geol. Soc. vol. ii. p. 160. 



