ORIGIN OF BARRIER REEFS AND ATOLLS. yy a | 
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The southern Maldives have deeper lagoons than the northern, 
fifty or sixty fathoms being found in them. This fact indi- 
cates that subsidence was probably most extensive to the 
south, and perhaps also most rapid. The sinking of the 
Chagos Bank, which lies farther to the south in nearly 
the same line, may therefore have had some connection with 
the subsidence of the Maldives. Other drowned atoll reefs, 
of similar character, exist in the China Sea and to the north 
of Madagascar. 
The submerged Macclesfield Bank, and the Tizard Bank 
five degrees farther south, have been described by W. J. L. 
Wharton! and Captain Aldrich? The Tizard Bank is 10 miles 
broad, has depths of 30 to 47 fathoms in the lagoon part, and 
4 to 10 fathoms over the border on which alone are growing 
corals; but the border extends to the surface in eight places 
and at three of them are islets. The Macclesfield Bank is 
70 by 40 miles in area; it is like the Tizard, but lies deeper, 
the lagoon in places being 40 to 60 fathoms under water and 
the margin 4 to 10 fathoms. The Saya de Malha Bank, east 
of northern Madagascar, measures two degrees across, has 
depths of 60 to 70 fathoms within the lagoon part, and a 
border on the north and northeast sides at a depth of 10 to 
17 fathoms, a portion of which comes within 8 to 9 fathoms 
of the surface. Darwin remarks on its close resemblance to 
reefs of the Chagos Bank. To the south is the submerged 
Nazareth Bank, and Cargados Carajos at the south end of 
a reef region common to the two; and to the northeast, the 
Seychelles, of great area. The Seychelles have some granite 
islands near the centre, but constitute otherwise a great sub- 
merged bank; on the western border are shoals 3 to 7 fath- 
1 Nature, 1888, Feb. 23. 
2 Bulletin of Hydrographic Department, London, for February, 1889. 
