CALCAREOUS FORMATIONS OF THE SOLOMON GROUP. 573 



it differs essentially from all the other deposits of the class. Four prevailing 

 kinds of these rocks may be distinguished. 



(1) A friable rock, containing from 5 to 20 per cent, of carbonate of 



lime, and displaying to the eye only the white specks of minute 

 Foraminiferous tests with a few of macroscopic size, such as Cris- 

 tellaria cultrata and Nodosaria soluta, Molluscan shells being rarely 

 observed. Rocks of this character form the masses of Treasury and 

 Ugi Islands ; and their composition is described in detail in the 

 sections treating of those islands (vide pages 547, 551, 552). 



(2) A very friable rock, containing from 30 to 35 per cent, of carbonate of 



lime, and inclosing in great numbers the shells of Pteropods, 

 Gasteropods, Lamellibranchs, together with otoliths of fish, simple 

 corals, and numbers of Foraminiferous tests, many of them 

 macroscopic. Eocks of this character largely compose Alu, the 

 principal island of the Shortlands, and are exposed in the low hills 

 in the rear of Choiseul Bay (for the detailed descriptions, see 

 pages 562, 567). 



(3) A hard grey fossiliferous limestone, containing usually about 60 per 



cent, of carbonate of lime, and much volcanic debris. It is chiefly 

 composed of the broken down fragments of corals and Lamelli- 

 branchiate shells, with calcareous Algae, and a few Foraminifera. 

 This rock is exposed in the lower courses of the Treasury Island 

 streams, where it gives rise to water-falls on account of its greater 

 hardness. Its further description is given on page 552. 



(4) Coarse rocks, composed of the fragments of volcanic and coral rocks 



in rounded grains. Occasionally larger fragments, together with 

 shells, are imbedded. These rocks occur on the northern slopes of 

 St Christoval, near the coast. 

 The Second Class includes those rocks which are largely composed of coral, 

 Molluscan shells, Foraminiferous tests, and calcareous Algae, with but a small 

 proportion of volcanic debris. The share that each of these four principal 

 constituents takes in the building up of the rock differs widely ; and on this 

 basis the following groups have been made. Yet to the great majority of these 

 rocks, if not to all, the term of coral limestone is quite applicable in its widest 

 sense. Probably, however, the term " coral-reef formation " would be 

 preferable, as including the variously composed rocks that are being formed in 

 connection with coral-reefs. Whether the rock is formed by the upward 

 growth of successive tiers of large masses of reef-corals, or whether it is 

 formed from the fragments of such corals broken off by the waves and mixed 

 with shells and other organisms in varying proportions, such as must be form- 

 ing on the outer slopes of reefs, it has the same coral origin. For the same 



