CALCAREOUS FORMATIONS OF THE SOLOMON" GROUP. 557 



lakes, and a continuation of the downward movement to the same extent 

 would transform the interior of the island into a large salt-water lagoon. In 

 my traverses across this basin-shaped interior I found evidences of the sea 

 having very recently occupied its lower levels. In the marshy ground in the 

 vicinity of the lakes I occasionally came upon the coral rock exposed in flat 

 surfaces resembling those of the ordinary reef-flat, whilst in the drier regions 

 the soil was mixed with calcareous sand and shell " debris ; " and numbers of 

 sea shells, of ancient appearance from prolonged exposure, lay on the surface 

 or were buried an inch or two beneath the soil. Many of these shells, 

 belonging to the Ostraeida?, Cardiadse, Neritidse, &c, were encrusted on their 

 inner surfaces by a dried calcareous mud. On opening one bivalve shell, with 

 the two valves in apposition, the ligament having decayed, I found its interior 

 to a great extent filled by this dried calcareous mud, which, from the impression 

 that it retained of the soft parts of the Mollusc, afforded proof that the shell- 

 cavity had become filled with this material before decomposition had set in. 



The lakes, I should have added, do not occupy the centre of the basin but 

 lie towards the western side of the island, the centre being marked by an 

 eminence of coral limestone, the level summit of which is elevated about 160 

 feet above the sea. In concluding my description of the interior of this island, 

 I will refer to the basin of Port Mary Harbour which lies opposite the break in 

 the elevated rim on the west coast. The shore-reefs, which skirt the circum- 

 ference of Santa Anna, here enclose a remarkable circular lagoon 700 to 800 

 yards in width, which, entered by a narrow passage, affords a snug anchorage 

 for ships during the S.E. trade. Its depth is about 100 feet (16 to 17 fathoms), 

 thus corresponding with the depth of the basin in the interior of the island, 

 from which it is separated by a barrier of coral detritus, calcareous sand, and 

 shells, that a depression of about 25 feet would, as above observed, completely 

 submerge. 



It was only by a detailed examination of this island that I was enabled to 

 learn its true structure. Contrary to my expectations I found that, instead of 

 being composed in mass of coral rock, this upraised atoll had been formed 

 around a submerged volcanic peak which had been first invested, in great part, 

 by a deposit of calcareous mud on which the reef-corals had commenced to 

 grow. 



The most interesting locality for the examination of the structure of the 

 island is situated on the north coast. A mass of crystalline granular volcanic 

 rock, an altered dolerite, here protrudes in situ 6 to 8 feet above the level of the 

 reef- flat, forming a conspicuous object on the shore, and covering a space 

 of 10 yards in horizontal extent. This ancient peak, if I may so term it, has 

 evidently been denuded by the waves at the present sea-level ; and in places 

 most exposed to the action of the sea its surface is formed of water- worn frag- 



