﻿110 PKOF. E. W. SKEATS ON THE [Feb. I905, 



III. Discussion of the Chemical Results. 



In considering the results of the foregoing analyses, two points 

 of especial interest arise : — 



(1) The mode of formation of masses of dolomite. 



(2) The significance of the presence or absence of insoluble residue in a 



limestone. 



The first point will be dealt with in considering the miueralogical 

 evidence. 



Sir John Murray's work in connection with the Challenger Ex- 

 pedition added largely to our knowledge of the nature, composition, 

 and mode of formation of the deposits which are now being laid down 

 on the sea-bottom. It was found that in all the deep-sea deposits 

 insoluble residue was present in amounts which rarely, if ever, fell 

 below 1 per cent., often reached 3 to 5 per cent., and occasionally 

 rose to as much as 20 per eeut. It is generally agreed that deep- 

 sea deposits rarely attain any great thickness, since they accumulate 

 with extreme slowness. The presence in deep waters of carbon- 

 dioxide under pressure determines the solution of much of the 

 calcareous parts of the tests of the minute organisms which form 

 the deposit, and as solution proceeds the proportion of insoluble to 

 calcareous material becomes greater. Erom external sources other 

 products, such as finely-divided volcanic material and cosmic dust, 

 also contribute to the amount of insoluble matter in deep-sea 

 formations. When a deposit composed of the skeletons of calcareous 

 organisms is being laid down near a coast formed of non-calcareous 

 rocks, finely-divided detrital matter intermingles with the calcareous 

 skeletons, so that the resulting limestone is rendered impure. 



Some thin fringing coral-reefs may be of this character. I 

 have recorded an analysis of such a reef-rock from Singatoka (Viti 

 Levu). 1 Shallow-water impure limestones may also be formed 

 by the intermingling, not of detrital matter, but of finely-divided 

 volcanic debris with the purely-calcareous material. Recent coral- 

 reefs growing in the vicinity of volcanoes ejecting ashes often have 

 such insoluble matter included within them. Mango, an upraised 

 coral-island in the Lau group of the Eijis, provides an example of 

 an impure limestone of this character. 2 



A chemically-pure limestone must have been deposited under 

 some such conditions as the following : — 



1. The material must have accumulated rapidly. If it had been 

 slowly deposited, solution of the calcareous parts of the organisms 

 and the raining-down of volcanic material and cosmic dust would 

 raise the percentage of insoluble residue. 



2. The deposit must have been laid down in shallow water, since 

 only under these conditions do calcareous organisms exist in sufficient 

 abundance to give rise to thick and rapidly-formed deposits. 



1 Bull. Mus. Comp. Zool. vol. xlii (1903) p. 79. 



2 Ibid. pp. 71-73. 



