204 



NA TURE 



[Dec. 31, T 885 



observations, there is an annual rainfall at the coast of 

 150 inches, that I rarely came upon a thickness of a 

 hundred feet of coral limestone. One of the corollaries 

 of the theory of subsidence is concerned with the great 

 thickness of atolls and barrier-reefs. My observations in 

 this region — and it is such regions that can alone afford 

 such evidence — show that atolls and barr'er-reefs can be 

 formed with no greater thickness than they would possess 

 in accordance with the depths in which reef-corals thrive, 

 the vertical tliickness of the reef not exceeding the depth 

 of the reef-coral zone. . . . The only objection worthy of 

 attention that had been advanced against the atoll-theory 

 of Mr. Darwin was, in the opinion of Sir Charles Lyell,' 

 the circumstance that, as far as was known, no bed or 

 formation of coral of any thickness had been discovered. 

 This objection, which was proposed by Mr. Maclaren in 

 1842, derives additional force at the present day in the 

 light of my observations in the Solomon Islands. 



3. T/iai these upraised reef-masses in the majority of 

 islands rest on a partially consolidated deposit which 

 possesses the characters of the " volcanic muds " ivhich 

 ■were found during the " Challenger" Expedition to he 

 at present forming around volcanic islands. 



4. That this deposit envelops anciently submerged 

 volcanic peaks. 



These two latter conclusions corroborate in a remark- 

 able manner the views, based on the observations of the 

 Challengrr Expedition, which Mr. Murray has advanced. 

 I will cite the structures of two islands to illustrate these 

 views. In the small island of Santa Catalina I found 

 that the elevated reef was based on volcanic rock with 

 the intervention of a thin brecciated conglomerate. In 

 the island of Treasury I found the volcanic rock covered 

 by a soft, partially consolidated volcanic mud, which 

 attained a thickness of some 300 or 400 feet, and was 

 itself incrusted on the lower slopes of the island by the 

 elevated reef-mass. In the one island, the volcanic peak 

 had been exposed to breaker-action before the reef-corals 

 established themselves. In the other island, the sub- 

 merged volcanic peak was first brought within the rccf- 

 coral zone by the deposition of layers of " volcanic mud ' 

 upon it, assisted by the movement of elevation. 



With reference to my own bias on this subject, I may 

 here add that during the first eighteen months I passed in 

 the Solomon Islands I was only acquainted with tlic 

 theory of subsidence, and that after having failed to make 

 my observations harmonise with the theory of Mr. Darwin, 

 I collected my facts with a very confused idea of the 

 direction towards which they were tending. It was there- 

 fore a cause of great satisfaction to myself when I first 

 became acquainted with the views held by Mr. Murray. 



These calcareous rocks, in the examination of which 

 Mr. Murray used the methods he employed in the case of 

 the deep-sea deposits, may be grouped into two chief 

 classes, according to the proportion of volcanic debris 

 they contain. 



The frst class comprises those rocks which, being 

 largely composed of volcanic debris mixed with the tests 

 of Foraminifera, Pteropods, and other iMollusks, have a 

 composition very similar to that of the volcanic muds at 

 present forming around oceanic volcanic islands in the 

 Pacific. These rocks contain both pelagic and bottom 

 forms of Foraminifera, and four prevailing kinds of them 

 may be distinguished. 



I. A friable rock, containing froin 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 microscopic size, entire Molluscan shells laeing rarely 

 embedded. The carbonate of lime consists of Cocco- 

 liths, Rhabdoliths, Gasteropod, and Lamellibranchiate 

 shells, Echinoderm fragments, calcareous Algs, and manv 

 pelagic and bottom forms of Foraminifera. The residue 

 consists for the most part of the minerals felspar, mag- 



• " Principles of Geology," 12th edit. vol. ii. p. 612 



netite, augite, hornblende, fragments of pumice, scoriaj, 

 and other volcanic rocks, with many glassy fragments, and 

 of a fine argillaceous matter which forms about a third of 

 the rock-substance. Rocks of this character form the 

 masses of Treasury and Ugi Islands. 



2. A very friable rock, containing from 30 to 35 per 

 cent, of carbonate of lime. These rocks resemble in 

 their general composition the rocks of the previous group, 

 but they difler in the circumstance that they inclose in 

 great numbers the entire shells of Pteropods, Gasteropods, 

 Lamellibranchiates, together with simple corals of deep- 

 sea genera, and the otoliths of fish. There are contained 

 in the residue, in addition to the mineral particles and 

 fine argillaceous material, a great many glauconitic-like 

 casts of Foraminifera. Rocks of this character largely 

 compose Alu, the principal island of the Shortland 

 Islands, and are exposed in the low hills in the rear of 

 Choiseul Bay. 



3. A hard, grey fossiliferous limestone, containing 

 usually about 60 per cent, of carbonate of lime and much 

 volcanic debris. Such a rock, which is exposed in the 

 lower courses of the Treasury streams, is chiefly composed 

 of the broken-down fragments of corals and Lamelli- 

 branchiate shells, with calcareous Alg» and a few 

 Foraminifera. 



4 Coarse-grained rocks composed of the fragments of 

 volcanic and coral rocks in rounded grains. Occasionally 

 larger fragments, together with shells, are imbedded. 

 Such 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 prin- 

 cipal constituents takes in the building up of the rock 

 differs widely, and on this basis the following groups 

 have been made. Whether the rock is mainly formed of 

 the massive corals, or whether it is composed of the frag- 

 ments of such corals broken off by the waves and mi.xed 

 with shells and other organisms in varying proportions, 

 such a rock as must be forming on the outer slopes of reefs, 

 or whether it is composed of the consolidated calcareous 

 muds and sands which are found at the bottom of lagoons, 

 it has in all cases the same coral origin. The variety in 

 character exhibited in the following groups of coral 

 limestones may be thus in a great measure explained. 



1. Coral rocks, properly so-called, which are merely 

 the massive reef-corals in different stages of fossili- 

 sation. 



2. Coral rocks, which are chiefly made up of calcareous 

 Algffi, fragments of Molluscan shells, corals, and Echino- 

 derms, the interstices being filled up by the tests of 

 Foraminifera and other small calcareous organisms. In 

 the composition of such rocks, which form the tnajority 

 of the so-called coral limestones in the Solomon Islands, 

 coral fragments take only a secondary part. The per- 

 centage of carbonate of lime in these rocks varies between 

 90 and 95, the residue consisting of the common volcanic 

 minerals, siliceous casts of Foraminifera and a fine 

 argillaceous matter. 



3. Chalk-like coral limestones, which contain about 95 

 per cent, of carbonate of lime, and are chiefly composed 

 of the fragments of Molluscan shells, Echinodernis, corals, 

 calcareous Alga;, and Foraminifera. These rocks, there- 

 fore, in their general composition resemble the rocks of 

 the second group of coral limestones ; but they differ 

 conspicuously in their chalk-like appearance and in being 

 more friable. They occupy the usual surface position of 

 other coral rocks, although not being of common occur- 

 rence. I found them overlying the soft Foraminiferous 

 and Pteropod deposit in the Shortland Islands, and they 

 may^ be sometimes found forming the central elevated 

 portions of existing reefs. One of the specimens of coral 

 this rock contained, according to a determination made 



