140 THE VOYAGE OF H.M.S. CHALLENGES. 



fathoms, as a cast of 1075 fathoms was obtained just east of the northwest part of the 

 depression and another of 1250 fathoms just west of it. 1 



Circumstances did not permit of the definition of the edge of the 100 fathom bank 

 on the northwest side of the Bermudas, but a cast of 1370 fathoms was obtained at a 

 distance of 6 miles from the rocks awash, and one of 2100 fathoms at a distance of 10 

 miles. The deepest sounding obtained close to the atoll was one of 1950 fathoms, 

 5ig miles west of the extreme west point of the rocks awash (see Sheet 8). 



Another important point to which attention was directed was the magnetic condition 

 of the islands. Observations made by the Governor, General Lefroy, at his official 

 residence, differed considerably from the Admiralty charts, and, consequently, instruc- 

 tions were received from the Hydrographer to ascertain whether those charts were in 

 error or not. The observations made by the Expedition showed that the variation 

 differed in various parts of the island as much as 6°, ranging from 4° W. to 10° W., the 

 smallest amount being found at a small islet just under the lighthouse on Gibb's Hill, 

 and the greatest at the point on the west side of Clarence Cove. The correct variation 

 was found by swinging the ship on all points of the compass, and ascertaining its errors 

 by azimuths of the sun, and the result so obtained agreed precisely with the Admiralty 

 chart. It does not appear that before the visit of the Expedition this peculiarity of the 

 Bermuda group was known, as the islands were said to consist entirely of calcareous rocks, 

 derived from comminuted shells and corals, although Lieutenant Nelson, R.E., noticed on 

 the island small pieces of oxide of iron of very questionable origin. It is, however, evident 

 from these observations 2 that some disturbing cause exists in the neighbourhood of the 

 islands which vitiates magnetic observations taken on shore. 



At a depth of 200 fathoms, about 2 miles from the reefs, the deposit was composed 

 of large fragments of Coral, Foraminifera, Echinoderms, Polyzoa, Molluscs, Algae, and 

 concretionary lumps, some of which were 2 or 3 centimetres in diameter. At 380 

 fathoms, 3 miles from the reefs, the fragments were smaller, and, in addition to the 

 above, there were many Pteropod and Heteropod shells. At 950 fathoms, 4 miles 

 from the reefs, the particles were still smaller, and there was a considerable admixture 

 of pelagic Foraminifera. At 1950 fathoms, 5 miles from the reefs, the deposit 

 was a nearly pure Globigerina ooze, made up chiefly of pelagic Foraminifera, with only a 

 small proportion of species living on the bottom, and fragments from the reefs. All these 

 deposits contained from 85 to 91 per cent, of carbonate of lime. The residue, after 

 treatment with weak acid, consisted of a few siliceous spicules, fragments of felspar, 

 augite, magnetite, and glassy rocks; none of the mineral particles exceeded - 07 mm. in 

 diameter. At 2G00 fathoms, 30 miles from the reef, the deposit was a Globigerina ooze, 

 containing only about 50 per cent, of carbonate of lime. 



1 Since this was written the outer bank, over which the least depth is 10 fathoms, has Been surveyed by H.M.S. 

 " Argus," and named after that vessel, the depth between it and the Challenger Bank being about 500 fathoms. 



2 Narr. Chall. Exp., vol. ii. p. 25, 1882. 



