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Darwin's theory, as is well known, is based on the fact that the temperature of the ocean at any considerable depth does not permit of the existence and luxuriant growth of the organisms that form the reefs. He himself estimated this limit of depth to be from 120 to 130 feet; Dana, as an extreme, 150 feet; while the recent very prolonged and successful investigations of Professor Alexander Agassiz in the Pacific and Indian Oceans lead him also to assign a limiting depth of 150 feet; the EFFECTIVE, REEF-FORMING CORALS, however, flourishing at a much smaller depth. Mr Stanley Gardiner gives for the most important reef-forming corals depths between 30 and 90 feet, while a few are found as low as 120 feet or even 180 feet.

It will thus be seen that the verdict of Funafuti is clearly and unmistakeably in favour of Darwin's theory. It is true that some zoologists find a difficulty in realising a slow sinking of parts of the ocean floor, and have suggested new and alternative explanations: but geologists generally, accepting the proofs of slow upheaval in some areas-- as shown by the admirable researches of Alexander Agassiz--consider that it is absolutely necessary to admit that this elevation is balanced by subsidence in other areas. If atolls and barrier-reefs did not exist we should indeed be at a great loss to frame a theory to account for their absence.

After finishing his book on Coral-reefs, Darwin made his summer excursion to North Wales, and prepared his important memoir on the glaciers of that district: but by October (1842) we find him fairly settled at work upon the second volume of his "Geology of the 'Beagle'--Geological Observations on the Volcanic Islands, visited during the Voyage of H.M.S. 'Beagle'". The whole of the year 1843 was devoted to this work, but he tells his friend Fox that he could "manage only a couple of hours per day, and that not very regularly." ("L.L." I. page 321.) Darwin's work on the various volcanic islands examined by him had given him the most intense pleasure, but the work of writing the book by the aid of his notes and specimens he found "uphill work," especially as he feared the book would not be read, "even by geologists." (Loc. cit.)

As a matter of fact the work is full of the most interesting observations and valuable suggestions, and the three editions (or reprints) which have appeared have proved a most valuable addition to geological literature. It is not necessary to refer to the novel and often very striking discoveries described in this well-known work. The subsidence beneath volcanic vents, the enormous denudation of volcanic cones reducing them to "basal wrecks," the effects of solfatarric action and the formation of various minerals in the cavities of rocks--all of these subjects find admirable illustration from his graphic descriptions. One of the most important discussions in this volume is that dealing with the "lamination" of lavas as especially well seen in the rocks of Ascension. Like Scrope, Darwin recognised the close analogy between the structure of these rocks and those of metamorphic origin--a subject which he followed out in the volume "Geological Observations on South America".

Of course in these days, since the application of the microscope to the study of rocks in thin sections, Darwin's nomenclature and descriptions of the petrological characters of the lavas appear to us somewhat crude. But it happened that the "Challenger" visited most of the volcanic islands described by Darwin, and the specimens brought home were examined by the eminent petrologist Professor Renard. Renard was so struck with the work done by Darwin, under disadvantageous conditions, that he undertook a translation of Darwin's work into French, and I cannot better indicate the manner in which the book is regarded by geologists than by quoting a passage from Renard's preface. Referring to his own work in studying the rocks brought home by the "Challenger" (Renard's descriptions of these rocks are contained in the "Challenger Reports". Mr Harker is supplementing these descriptions by a series of petrological memoirs on Darwin's specimens, the first of which appeared in the "Geological Magazine" for March, 1907.), he says:

"Je dus, en me livrant a ces recherches, suivre ligne par ligne les divers chapitres des "Observations geologiques" consacrees aux iles de l'Atlantique, oblige que j'etais de comparer d'une maniere suivie les resultats auxquels j'etais conduit avec ceux de Darwin, qui servaient de controle a mes constatations. Je ne tardai pas a eprouver une vive admiration pour ce chercheur qui, sans autre appareil que la loupe, sans autre reaction que quelques essais pyrognostiques, plus rarement quelques mesures au goniometre, parvenait a discerner la nature des agregats mineralogiques les plue complexes et les plus varies. Ce coup d'oeil qui savait embrasser de si vastes horizons, penetre ici profondement tous les details lithologiques. Avec quelle surete et quelle exactitude la structure et la composition des roches ne sont'elles pas determinees, l'origne de ces masses minerales deduite et confirmee par l'etude comparee des manifestations volcaniques d'autres regions; avec quelle science les relations entre les faits qu'il decouvre et ceux signales ailleurs par ses devanciers ne sont'elles pas etablies, et comme voici ebranlees les hypotheses regnantes, admises sans preuves, celles, par exemple, des crateres de soulevement et de la differenciation radicale des phenomenes plutoniques et volcaniques! Ce qui acheve de donner a ce livre un incomparable merite, ce sont les idees nouvelles qui s'y trouvent en germe et jetees la comme au hasard ainsi qu'un superflu d'abondance intellectuelle inepuisable." ("Observations Geologiques sur les Iles Volcaniques...", Paris, 1902, pages vi., vii.)

While engaged in his study of banded lavas, Darwin was struck with the analogy of their structure with that of glacier ice, and a note on the subject, in the form of a letter addressed to Professor J.D. Forbes, was published in the "Proceedings of the Royal Society of Edinburgh". (Vol. II. (1844-5), pages 17, 18.)

From April, 1832, to September, 1835, Darwin had been occupied in examining the coast or making inland journeys in the interior of the South American continent. Thus while eighteen months were devoted, at the beginning and end of the voyage to the study of volcanic islands and coral-reefs, no less than three and a half years were given to South American geology. The heavy task of dealing with the notes and specimens accumulated during that long period was left by Darwin to the last. Finishing the "Volcanic Islands" on February 14th, 1844, he, in July of the same year, commenced the preparation of two important works which engaged him till near the end of the year 1846. The first was his "Geological Observations on South America", the second a recast of his "Journal", published under the short title of "A Naturalist's Voyage round the World".