32 ON CORAL REEFS AND ISLANDS. 



tus. This was the result of Captain Beechey's investigations j 

 and those of the Expedition, though few, correspond. It is 

 however probable that deeper soundings would be found in the 

 large island of Nairsa (Dean's). In the Tarawan Group, south- 

 east of the Carolines, the depth, where examined by the Expedi- 

 tion, varied from 2 to 35 fathoms. Mr. Darwin found the latter 

 depth at Keeling's Island. Chamisso found 25 to 35 fathoms at 

 the Marshall Islands. 



The bottom of these large lagoons is very nearly uniform, va- 

 rying but little except from the occasional abrupt shallowings 

 produced by growing patches of reef. Soundings bring up sand, 

 pebbles, shells, and coral mud ; and the last mentioned material 

 appears to be quite common, even in lagoons of considerable 

 size. It has the same character as above described. The blu- 

 ish clay-like mud of the harbor of Tpngatabu may be classed 

 with these deposits.* It appears, therefore, that the finer coral 

 material of the shores prevails throughout the depths of the la- 

 goon. The growing reefs within the lagoons, are in the condi- 

 tion of the inner reefs about high islands. The corals grow but 

 little disturbed by the waves, and the reef-rock often contains 

 them in the position of growth. At Taputeouea (Kingsmills or 

 Tarawan Group), reefs very similar to those of the Feejees oc- 

 cur : they present the same large Astreeas 10 to 12 feet in diame- 

 ter, which once were growing where they stand but are now a 

 part of the solid lifeless rock. 



Beach formations of coral sandrock are common on the coral 

 islands, and they present the same features in every respect, as 

 those described. They were observed among the Paumotus, on 

 Karaka, Honden, Kawehe, and other islands. The stratified 

 character is always distinct and the layers slope toward the water 

 at the usual small angle, amounting to 5-7 degrees bordering 

 the lagoon, and 7-8 degrees on the seashore side of the land. 

 They often occupy a breadth of 30 to 50 yards, appearing like a 

 series of outcrops ; yet not unfrequently they are mostly concealed 

 by the sands of the beach,f and it is probable that they generally 

 underlie the loose surface material of the land. The rock is a 

 fine or coarse sandrock, or a coral pudding-stone, and consists 

 of beach materials. Occasionally it is quite compact, and resem- 

 bles common limestone, excepting in its whiter color ; but gen- 

 erally its sand origin is very apparent. 



The drift sandrock was not met with by the writer on any 

 coral islands visited, and probably for the reason that opportuni- 



* Darwin describes this mud as occurring at the Maldives, and at Keeling Island, 

 (op. cit. p. 26 ;) Kotzebue mentions it as common at the Marshall Atolls, and Lieu- 

 tenant Nelson observed it at the Bermuda's. 



f On the northern atolls of the Maldives, the beach sandrock is said to be quar- 

 ried out in square blocks and used for building. — Journ. Geog. Soc, v, 400. 



