72 THE GREAT BARRIER REEF. 



Plate XII. and many other of the most characteristic reef-views illustrated in this volume. In 

 "The Depths of the Sea," by the late Sir C. Wyville Thomson, who gives an account of the general 

 results of the dredging cruises of H.M.S. Porcupine and Lightning, during the summers of 1868, 

 1869, and 1870, the occurrence of Lophobclia prolifera, accompanied by more sparing growths of 

 as man}' as five species of Amphihelia, is reported as occurring in regular banks along the west 

 coasts of Scotland and of Ireland at depths varying from one hundred and fift)' to five hundred 

 fathoms. In another paragraph, p. 168, of the same work, the first-named type, Lophohelia, is 

 described as " forming stony copses which cover the bottom of the sea for many miles." In the 

 dredging cruise of Mr. Marshall Hall's yacht. Noma, in the month of May, 1870, in which the 

 author, then attached to the Natural History Departments of the British Museum, participated, 

 both Lophohelia and Amphihelia were obtained in abundance in depths of five hundred and six 

 hundred fathoms, off Setubal, on the coast of Portugal. Associated with these were robust tree- 

 like growths of the Eupsammian type, Dcndrophyllia ramea, whose ramifying coralla averaged 

 three or four feet in height, and were as massive and weighty as any of the most solid branching 

 Madreporae of tiie Great Barrier Reef. It is worthy of remark that this genus Dcndro- 

 phyllia is one of the representative Barrier Reef genera. It is illustrated in Chromo plate 

 No. VIII. by a form whose orange and scarlet polyps are indistinguishable in shape and colour 

 from those of the deep-sea type. 



A question very naturally arises, at this point, as to how it is that, with such an abun- 

 dant development of Stony-corals at these profound depths, coral-reefs are not formed in deep 

 water. Were such a thing possible, the western coast-lines of the British Isles and the Iberian 

 Peninsula would be as thickly girdled with coral-reefs as those of India or Australia. A logical 

 interpretation of their absence can be found onl}' in the fact, to which scarcely sufficient pro- 

 minence has been hitherto given, that the solid coral-rock, of which, as explained in a previous 

 page, the greater mass of all coral-reefs is composed, is constructed, exclusively, in tropical 

 areas subject to alternations of complete submersion and atmospheric exposure with the rise 

 and fall of the tide. It is necessarily conceded that the massive dome-shaped corals, such as 

 the Poritidae and Astreaceae, which are entirely absent in abyssal depths, contribute substantially 

 towards the formation of the generality of reefs. However, it needs but a glance at the 

 photographic reef-views reproduced in this volume, — notably at those of Plate V., No. 1, and 

 Plates IX., XL, and XII., — to demonstrate the fact that the above-named massive-growing 

 corals may be very sparingly represented, or even altogether absent, throughout reef-areas of 

 unlimited extent. The thicket-growths of the deep-sea Lophohelia, Amphihelia, and Dcndro- 

 phyllia would act with equal efficiency as reef-builders if they were capable of translation to 

 this higher, tidally-affected, plane. The solid coral-rock, which represents the chief constituent 

 of all reef-masses, is, as previously remarked, composed exclusively of the broken-down, more 

 or less triturated, and subsequently re-consolidated, calcareous elements of the peripherally 



