STRUCTURE OF CORAL ISLAXDS. 149 



The bottom of these large lagoons is very nearly uniform, 

 varying but little except from the occasional abrupt shallovvings 

 l)roduced 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 bluish clay-like mud of the harbour of 

 Tongatabu may be classed with these deposits. Darwin de- 

 scribes this mud as occurring at the Maldives, and at Reeling's . 

 Island (op. cit. p. 26) ; Kotzebue mentions it as common at the 

 Marshall atolls, and Lieutenant Nelson observed it at the 

 Bermudas. It appears, therefore, that the finer coral material of 

 the shores prevails throughout the depths of the lagoon. The 

 growing reefs within the lagoons are in the condition of the 

 ijuier 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 (Kingsmilfs or 

 Gilbert's Group), reefs very similar to those of the Feejees 

 occur; they contain similar large Astrseas ten to twelve feet in 

 diameter, which once were growing where they stand, but are 

 now a part of the solid lifeless rock. 



Beach formations of coral sand-rock 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 Raraka, Honden, Rawehe, and other islands. The strati- 

 fied character is ahvays distinct, and the layers slope toward the 

 water at the usual small angle, amounting to 5 — 7 degrees 

 bordering the lagoon, and 6 — 8 degrees on the seashore side of 

 the land. Agassiz gives the same angle for the seaward slope 

 of similar deposits at Rey West. The rock is largely a fine 

 oolite. They often occupy a breadth of thirty to fifiy yards, 

 appearing hke a series of outcrops ; yet they are frequently 

 covered by the sands of the steep part of the beach. It is 

 jjrobable that they generally underlie the loose surface material 

 of the land. The rock is a fine or coarse sand-rock, or an 

 oolite, or a coral pudding-stone, and consists of beach materials. 

 Occasionally it is quite compact, and resembles common lime- 



