PHYSICAL HISTORY AND GEOLOGY. 25 



way communicating with Castle Harbor we sounded nine 

 fathoms in the water back of the reef, and I believe that 

 this depth, or even a considerably greater one, must be found 

 in many places. On the outer side the depth of water increases 

 more rapidly, but not in a degree as to indicate abrupt- 

 ness of descent. The organic growth, which is serpuloid super- 

 ficially, comes to the surface in discontinuous patches, over 

 whose line a white surf may be seen breaking during the 

 greater part of the day. These are the " boilers," or secondary 

 atolls, as they have been sometimes called. 



The outer soundings made by the officers of the " Challenger" 

 indicate a gradual descent of the bottom for a distance of 

 about a mile, when a much more abrupt slope begins. It is 

 claimed that within the first belt the average depth does not 

 exceed 12 fathoms, but we sounded 13 fathoms, after making 

 full allowance for slip, at a distance of not more than 300 feet 

 from the breaking surf. Our facilities, however, did* not per- 

 mit us to make extended observations in this direction, nor 

 was the condition of the water, when we crossed over the reef, 

 favorable for such observations. At a point seven miles from 

 the northern reef the " Challenger" obtained the extraordinary 

 depth of 12,000 feet, which would give an average descent of 

 slope of one in three, exceeding probably that of any equal 

 slope on the land surface. Drained of its surrounding waters, 

 the Bermudas would, from this side, appear like a stupendous 

 tower, in comparison with which the principal mountain 

 peaks of the land would, as far as abruptness of slope is con- 

 cerned, sink into insignificance. Yet it would seem that, in a 

 general way, at a distance of ten miles in nearly all directions 

 the depth is only from 9,000 to 13,000 feet, or not more than 

 that which is found at an equal distance off the sub-continental 

 plateau. Twenty miles to the south west-by-west of the Ber- 

 mudas two submerged banks come to within 20-50 fathoms of 

 the surface, and seemingly represent the continuation of a 

 range of heights of which the Bermudas constitute a section. 

 But not impossibly they are merely the summits of isolated 



