292 CORALS AND CORAL ISLANDS, 
The wide oceans are, however, wonderfully free from 
banks approaching the surface within one hundred fathoms. 
The Indian Ocean and the China seas afford exceptions. 
But the principal submerged banks of these waters have 
the lagoon-basins and raised borders of an atoll and thus 
bear, as has been explained (page 272), positive evidence of 
a former atoll existence and therefore of subsidence. They 
illustrate the fact ‘that the rate of subsidence has not always 
been within the narrow maximum limit that is favorable to 
‘the atoll, but sometimes has exceeded it to their destruction. 
The regions of such sunken atolls in the China seas and 
Indian Ocean have many spots of shallow soundings which 
were probably sunk at the same time. 
Mr. Murray observes that “the soundings of the ‘Tus- 
carora’ and ‘Challenger’ have made known numerous sub- 
marine elevations; mountains rising from the general level 
of the ocean’s bed at a depth of 2,500 or 5,000 fathoms up 
to within a few hundred fathoms of the surface.” But “a 
few hundred fathoms,” if we make “few” equal two means 
twelve hundred feet or more, which leaves a long interval 
unfilled.’ 
The atoll of the Tortugas, and others in the West Indies, 
are regarded by Mr. Agassiz as having a basement, up to the 
coral-growing limit, of pelagic limestone or of some other 
material. It may be so; but there is as yet no proof of it. 
1 The actual depths over the elevations in the “ Tuscarora” section between the 
Hawaiian Islands and Japan, numbering them from east to west are as follows: (1) 
11,500 feet; (2) 7,500 feet; (3) 8,400 feet; (4) 12,000; (5) 9,000 (this seven miles 
west of Marcus Island) ; (6) 9,600 feet. Whether ridges or peaks, the facts do not 
decide; probably the former. No. 1 has a base of 185 miles with the mean east- 
ward slope 40 feet per mile (= 1: 132) and the westward 128 feet per mile. No. 2 
has a breadth of 396 miles, with the mean eastern slope mostly 37 feet per mile, 
but 51 feet toward the top, and the westward 55 feet per mile (1:96). No.3 
was the narrowest and steepest, it being about 100 miles broad at base, and having 
the mean eastern slope 192 feet per mile and the mean western 200 feet. 
