DARWIN ON CORAL REEFS. 389 



face level of the sea, in no one instance emerge above it. To escape this latter 

 most improbable admission, which implies the existence of submarine chains of 

 mountains of almost the same height extending over areas of many thousand 

 square miles, there is but one alternative, namely, the prolonged subsidence of 

 the foundations on which the atolls were primarily based, together with the 

 upward growth of the reef-constructing corals. On this view every difficulty 

 vanishes ; fringing reefs are thus converted in barrier reefs, and barrier reefs, 

 when encircling islands, are thus converted into atolls the instant the last pin- 

 nacle of land sinks beneath the surface- of the ocean. 



" Finally, when the two great types of structure — the barrier reefs and 

 atolls on the one hand, and fringing-reefs on the other — are laid down in 

 colours on the map, a magnificent and harmonious picture of the movements 

 which the crust of the earth has within a late period undergone, is presented to 

 us. We there see vast areas rising, with volcanic matter every now and then 

 bursting forth through the vents or fissures with which they are traversed. We 

 see other wide spaces slowly sinking without any volcanic outbursts, and we 

 may feel sure that this sinking must have been immense in amount as well as 

 in area, thus to have buried, over the broad face of the ocean, every one of those 

 mountains above which atolls now stand, like monuments, marking the place of 

 their former existence. Reflecting how powerful an agent with respect to de- 

 nudation, and consequently to the nature and thickness of the deposits ia 

 accumulation, the sea must ever be when acting for prolonged periods on the 

 land during either its slow emergence or subsidence : reflecting also on the 

 final effects of these movements in the interchange of land and ocean- water on 

 the climate of the earth, and on the distribution of organic beings, I may be 

 permitted to hope, that the conclusions derived from the study of coral forma- 

 tions, originally attempted merely to explain their peculiar forms, may be 

 thought worthy of the attention of geologists." Pages 146 — 148. 



D. T. A. 



II. Travels in North America, with Geological Observations on 

 the United States, Canada, and Nova Scotia. By Charles 

 Lyell, Esq. F. R. S. 2 Vols. 12mo. pp. 588. Map and Plates. 



So far as regards geology, Mr. Lyell's work is eminently valu- 

 able, since it presents a connected view of the results of a large 

 number of careful surveys of different parts of the Continent of 

 North America, by means of a coloured geological map, in which 

 the whole amount of information at present known on the subject 

 of North American geology is incorporated. The importance of 

 this as a means of simplifying and generalising the notions of 

 English geologists with regard to the succession of strata on 

 the other side of the Atlantic, it would be difficult to estimate too 

 highly. 



Mr. Lyell has also added much to the knowledge hitherto 

 possessed on the subject of American geology by his own investi- 

 gations in the field, and in the present notice we propose to point 

 out in order the various geological matters touched on in the 

 work before us, commencing with the older rocks, and so ap- 

 proaching last of all to those of new er date. The latter indeed, 

 although perhaps the most important, we shall here scarcely 

 allude to, because, having formed the subject of communications to 

 the Geological Society, they either have been already or will be 

 hereafter described in greater detail than even in the book itself 



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