of Edinburgh, Session 1885 - 86 . 
441 
the south-west of the island. These hanks seem to exactly resemble 
the “Dacia” and “Seine” hanks. The minimum depth on the 
“ Challenger ” hank is 24 fathoms, and on the “ Argus ” hank, 
which lies farthest to the southward, is 10 fathoms. The separat- 
ing channels have a depth of 1000 and 600 fathoms respectively. 
On the reef, which surrounds the island of Bermuda, the water 
deepens very slowly to 25 fathoms. As soon as this depth is 
passed, the declivity becomes very steep, and is no douht in many 
places precipitous. The mean angle of slope for depth-intervals 
between 34 and 344 fathoms is 25° '3, between 29 and 239 fathoms 
it is 26°-4, and between 24 and 155 fathoms it is 27°. The western 
and northern sides have a steeper average slope than the southern 
and eastern ones. On the western side we have, from 42 fathoms 
on the reef to 1950 fathoms, the enormous average slope of 29°; and 
on the north side of the “ Challenger ” bank, between the depths 
175 and 1250 fathoms, we have an angle of 40°. It is difficult to 
believe that these average slopes are not made up of large portions 
which are really vertical, with gentler slopes at greater depths. The 
closer soundings on the “Dacia” bank and Coral patch revealed 
such precipices. 
While the “Challenger” was at Tahiti, very careful soundings 
were made to determine the actual slope of the reef. At a distance 
of 125 fathoms from the edge of the reef, a depth of 30 to 35 
fathoms is found; at 150 fathoms from the reef the depth is from 
90 to 100 fathoms. In this interval the slope attains its maximum 
angle of from 60° to 70°. “ It is believed to have been formed by 
huge masses and heads of coral which have been torn away from 
the ledge between the edge of the reef and 35 fathoms during 
storms, or by overhanging masses which have fallen by their own 
weight. In this way a talus has been formed on which the corals, 
living down to 35 fathoms, have found a foundation on which to 
build farther seawards, for this seaward slope is the great growing 
surface of the reef.”* If the steepest part of the slope is a talus, it 
is evident that the solid rock face inside the talus must be still 
steeper, and a slope steeper than 70° is a precipice. 
* Challenger Narrative, vol. i. p. 781. 
