36 
ATOLLS. 
Cn. L 
is only a little less. Within the Maldiva atolls there 
are large spaces with 45 fathoms, and some soundings 
are laid down at 49 fathoms. The greater part of the 
bottom in most lagoons, is formed of sediment ; large 
spaces have exactly the same depth, or the depth 
varies so insensibly, that it is evident that no other 
means excepting aqueous deposition, could have 
levelled the surface so equally. In the Maldiva atolls 
this is very conspicuous, and likewise in some of the 
Caroline and Marshall Islands. In the former, large 
spaces consist of sand and soft clay/, and Kotzebue 
speaks of clay having been found within one of the 
Marshall atolls. No doubt this clay is calcareous mud, 
similar to that at Keeling Island, and to that at Ber- 
muda already referred to, as undistinguishable from 
disintegrated chalk, and which Lieut. Nelson says is 
called there pipe-clay. 1 
Where the waves act with unequal force on the 
two sides of an atoll, the islets appear to be first 
formed, and are generally of greater length on the more 
exposed shore. The islets, also, which are placed 
1 I may here observe that on the coast of Brazil, where there is 
much coral, the soundings near the land are described by Admiral 
Boussin, in the Pilote du Brisil, as siliceous sand, mingled with 
much finely comminuted particles of shells and coral. Further in 
the offing, for a space of 1,300 miles along the coast, from the 
Abrolhos islands to Maranham, the bottom in many places is com- 
posed of ‘ tuf blanc, mele ou formd de madrepores broy6s.’ This 
white substance, probably is analogous to that which occurs within 
the above-mentioned lagoons; it is sometimes, according to Boussin, 
firm, and he compares it to mortar. [Probably the clay is commonly 
similar to that mentioned by Mr. Guppy (Proc. B. S. Edin. vol. xiii. 
p. 879 n.) and others. See the abstract of his paper in Appendix II.] 
