

NARRATIVE OF THE CRUISE. 139 



depth is 2 or 3 feet. The bottom is covered with a fine calcareous sand, and in some 

 parts of the lagoons living Actiniaria, Hydroids, &c, are found growing. These little 

 atolls were evidently formed without subsidence, and at once suggested the possibility 

 that the larger atolls of the Pacific and Indian Oceans might have been formed by a 

 somewhat similar mode of growth. 



Attention was directed during the stay to defining, as far as practicable, the edges 

 and slope of the bank or atoll, carrying the soundings down to 1500 or 2000 fathoms, 

 in fact, to oceanic depths. No information on these points had as yet been ascertained, 

 but it was believed— from the fact of H.M.S. "Ariadne" having obtained a cast of 11^ 

 fathoms 4 miles from the breakers or rocks awash, and from the statement that banks 

 existed in a S.W. direction from the island, 1 which had been surveyed by H.M.S. 

 "Columbine" in 1829 — that the bank on which the islands and reefs are situated was 

 really of much larger extent than was generally supposed. By the kindness of Captain 

 Aplin, then in charge of the dockyard, who placed one of the yard tugs at the disposal 

 of the Expedition, and gave assistance in other ways, it was possible to commence this 

 work at once. It was found that on the southeast edge of the bank the 100 fathom line 

 of soundings was at an average distance of 1^ miles from the rocks awash, and that 

 the depth increased rapidly from 30 to 350 or 400 fathoms, the slope being at an angle 

 of about 20° from the horizontal, but from that depth to 1000 fathoms the slope varied 

 from 7° to 15° from the horizontal. On the northeast edge of the bank the 100 fathom 

 line of soundings was at an average distance of 3 miles from the rocks awash, and 

 the slope was much more gradual. On the southwest side of the bank the 100 fathom 

 line of soundings appears to extend at one point nearly 5 miles from the rocks awash. 



About 4 miles southwest of the southwest extremity of the 100 fathom edge of the 

 Bermuda Bank the Challenger sounded and anchored on the " inner bank " of the 

 " Columbine," in 30 fathoms, with Gibb's Hill lighthouse, N. 54° 14' E. (true), distant 13 

 miles. The boats were employed one day in obtaining soundings on this bank, but 

 owing to the rough weather rendering the men sick, 2 and to the barometer falling, the 

 officers were unable to define its limits or to look for the " outer shoal," out of sight of 

 land, on which the " Columbine" anchored in 1820, and on which soundings were also 

 taken by the " Larne " in 183G. From the depths obtained, the inner or Challenger 

 Bank appears to be of some extent, certainly not less than 10 miles in circumference, the 

 shallowest water found being 24 fathoms, and it is quite possible that it joins the outer or 

 " Columbine " Bank, or that at any rate the depths between the two do not much exceed 

 100 fathoms. 



In the depression, 3^ miles wide, between the Challenger Bank and the southwest 

 extremity of the Bermuda Bank, the soundings, in all probability, do not exceed 1000 



1 Dana, Corals and Coral Islands, London, 1872 ; Findlay, North Atlantic Memoir, London, 1856. 



2 Seamen unused to the work of surveying ships are very often sea-sick in boats. 



