CALCAKEOUS FORMATIONS OF THE SOLOMON GROUP. 575 



carbonate in this group varies between 90 and 95, the residue con- 

 sisting of the minerals named in the subjoined analysis, of 

 Foraminiferous casts, and argillaceous matter. 



The Composition of a Typical Sample. 



Carbonate of Calcium (94 - 47 per cent.), consisting of the various 

 materials mentioned above. 



Residue (5*53), consisting of — 



(a) Minerals (l/OO) m. di. - 2 mm.; augite, hornblende, felspar, 



magnetite, and a few glassy fragments. 



(b) Siliceous Organisms (2 , 00), casts of Foraminifera, &c, of a 



red colour. 



(c) Fine Washings (2*53), broken down parts of casts of 



Foraminifera, fine mineral particles, argillaceous matter, 

 and a red material — probably oxide of iron. 



(3) Compact fawn-coloured crystalline limestones of a homogeneous 



texture, apparently formed by the consolidation of the ooze found 

 at the bottom of the lagoons inside coral-reefs. These rocks are of 

 common occurrence on the lower slopes of the large island of St 

 Christoval, where they overlie the volcanic rocks of the district. 

 They are sometimes coarse-grained. 



(4) Chalk-like coral limestones, which contain about 95 per cent, of 



calcium carbonate, and are chiefly composed of the fragments of 

 Molluscan shells, Echinoderms, corals, calcareous Algse, and Fora- 

 minifera. These rocks, therefore, in their general composition 

 resemble the rocks of the second group of coral limestone 

 differ conspicuously in their chalk-like appearance and in being 

 more friable. They occupy, however, the usual surface position of 

 other coral rocks (as shown in the description of the Shortland 

 Islands, page 563), although being by no means of common occurrence. 

 They may sometimes be found in the central elevated portions of 

 islands like Biu, which have been formed by the partial upheaval of 

 coral reefs. I found them only on four occasions in the Solomon 

 group, viz., at Alu, Choiseul Bay, the Three Sisters, and Biu Island 

 Their composition is described at length in pages 563, 568. 



(5) The Foraminiferal limestones, which are grey or yellowish-brown in 



colour, hard and compact in texture, and are chiefly made up of the 

 tests of pelagic and bottom-living Foraminifera. They contain 

 generally from 75 to 85 per cent, of calcium carbonate, their typical 



* White men resident in the group tell me that chalk-like rocks are found in the island of Ulaua, 

 which lies near the Three Sisters. 



