86 THE GREAT BARRIER REEF. 



rocks, such as St. Christoval, which, although over 4,000 feet in height, showed no calcareous envelopes of a greater height 

 than 500 feet above the sea, the coral limestone crust being even thinner than at the smaller and more recent islands. 

 From these considerations Mr. Guppy concludes (i) that these upraised reef-masses, whether atoll, barrier reef, or fring- 

 ing-reef, were formed in a region of elevation ; (2) that such upraised reefs are of moderate thickness, their virtual measure- 

 ment not exceeding the limit of the depth of the coral-reef zone — i.e., not more than about 150 feet ; (3) that these up- 

 raised reef-masses in the majority of islands rest on a partially consolidated deposit which possesses the characters of the 

 ' volcanic muds,' which were found during the Challenger expedition to be at present forming around volcanic islands ; 

 (4) that this deposit envelops anciently submerged volcanic peaks." 



An important communication bearing on Dr. IVIurray's new theory of atoll and barrier- 

 reef fonnation is contributed by Captain Wharton, R.N., Hydrographer to the Admiralty, to 

 the columns of Nature of Feb. 23, 1888. The article supports Mr. Murray's views in as 

 tar as it expresses the writer's opinion that the later evidence on coral growth justifies an 

 abandonment of the supposition that subsidence plays a principal part in the production of 

 barrier reefs and atolls ; but, at the same time, it opposes Dr. Murray's theory that the 

 disintegration and solution of dead coral by the chemical action of sea-water is a primary 

 factor in the hollowing and deepening of the lagoons that characterised the interiors of both 

 these classes of reefs. The Tizard and Macclesfield Banks in the China seas are specially 

 adduced in this association. Respecting the former, Captain Wharton writes : " The Tizard 

 Bank, in lat. 10° 20" N., and long. 114° 25" E., is thirty-two nautical miles in length with 

 an extreme breadth of ten miles, and was well surveyed in 1867. The central portion is very 

 fiat and almost void of patches. Its depth is from thirty to forty-seven fathoms. Its edge is 

 crowned with a coral rim varying from four to ten fathoms in depth, broken here and there 

 by openings, in some cases over thirty fathoms deep. The rim is composed of coral in luxuriant 

 growth, and it can scarcely be doubted that in time it will reach the surface. In fact, on its 

 periphery of lOO miles, in eight places small patches of reef, three of which bear islets, have 

 already done so. When the remaining portions of the rim are also awash, the reef will be 

 in all respects an atoll similar to the great Madive atolls, without any necessity for solvent 

 action enlarging or deepening it. 



"The great Macclesfield Bank, further north, over seventy miles in length and forty miles in 

 width, is of precisely the same nature, but its development is not so far advanced ; the rim 

 being in no spot nearer the surface than ten fathoms, the water on it varying from that 

 amount to nineteen fathoms, while the depth of the enclosed area is from forty to sixt^' fathoms. 

 The survey of this bank is not so complete as in the case of some others, but enough 

 has been done to show its character very plainly. 



" How, precisely, it comes about that coral is growing in the yet deep rims of these 

 large banks and that little or none is fiourishing in the interior, evidence is yet wanting 

 to show. These, however, are the facts, and the result, so far as the necessit}' for further 



