Dec. 31, 1885] 



NA TURE 



only occasional fragments occur in the red argillaceous 

 soil. The greatest elevation at which I found the coral 

 rock was about 425 feet above the sea. The soft Forami- 

 niferous deposit, which forms the mass of the island, is 

 regularly bedded, the dip varying usually between 10' and 

 15°, but it may rise to as much as 35°. Entire shells are 

 rarely found in these beds, the Foraminiferous tests being 

 usually the only organic remains visible to the naked 

 eye. 



The island of Treasury affords an example of the next 

 type of island. It is oval in shape, has a length of nine 

 miles, and rises about 11 50 feet above the sea. Here we 

 have exposed the nucleus of volcanic rock which has 

 been covered over by soft bedded deposits that resemble, 

 like those of Ugi,'lhe muds found in the Challenger 

 Expedition to be at present forming around oceanic 

 volcanic islands, whilst the coral limestone only attains 

 any thickness near the coast, and is wanting altogether in 

 the higher regions of the island. At elevations exceeding 

 400 feet above the sea the coral rock generally disappears 

 from the surface. Above this height it is only found oc- 

 casionally, 900 feet being the greatest elevation at which 

 I found a fragment. The thickness of the coral limestone 

 does not exceed 100 feet. The soft deposit, which is 

 regularly bedded, the dip varying between 10° and 30", 

 displays a greater variety in its characters than the similar 

 deposit in the island of Ugi. As a rule it presents to the 

 naked eye no other conspicuous organic remains than the 

 white specks of the more minute Foraminiferous tests and 

 the larger microscopic tests of such species as Cristellaria 

 calcar, C. mamilligera, and others ; but in some localities 

 this deposit becomes highly fo^siliferous, when it assumes 

 a more compact texture, and displays to the eye fragments 

 of corals with Ptcropod and Lamellibranchiate shells. As 

 shown in the accompanying diagram, the structural history 



Soh Foraminiferous deposit^ 



Ancient volcanic peakj 

 Cora! limestone 



Ideal section of an island displ aying the originally-submerged volcanic 

 peak, the overlying soft deposits, and the encrusting coral limestone. 



of this island of Treasury may be readily inferred. An 

 ancient submerged volcanic peak, having been covered by 

 a thickness of some hundreds of feet of deposits, for the 

 most part resembling the muds now being formed around 

 volcanic islands, has by this means and by the movement 

 of elevation been brought up to the zone of reef-building 

 corals. After the: coral reefs had become established, the 

 whole structure e.xperienced an upheaval of nearly 1200 

 feet. 



In the island of Alu, the principal of the Shortland 

 Islands, another type of structure is exhibited. This 

 island, which has a breadth of eleven or twelve miles and 

 an elevation of about 500 feet, is composed in its north- 

 west portion of ancient and originally deep-seated volcanic 

 rocks (mostly quartz-diorites), while the greater part of it, 

 together with the off-lying lesser islands and islets, is made 

 up of more recent calcareous formations. In describing 

 its structural history 1 shall be describing its structure. 

 We have the original land of volcanic formation in the 

 north-west part of the island, from which, as from a neu- 

 cleus, line after line of barrier-reef has been advanced in 

 a south-easterly direction based on a foundation of 

 Pteropodand Foraminiferous muds,and formingultimately, 

 as the upheaving movement continued, the large island of 

 Alu, which yet preserves iu the ridges of its interior these 

 ancient barrier-reefs now removed far from the coast and 

 elevated some hundreds of feet above the present sea- 

 level. The soft deposit underlying the elevated reef- 

 masses contains in abundance the shells of Pteropods and 

 bivalves, the otoliths of fish, the tests of pelagic and 

 bottom-living Foraminifera, and some simple corals of 



deep-sea genera. The overlying coral limestone some- 

 times assumes a chalk-like character, and in the interior 

 of the island it may give place to a Foraminiferal lime- 

 stone. The characters of these different rocks are de- 

 scribed in the second part of this paper. The thickness 

 of the coral limestones in this island is probably under 

 100 feet. When it caps the upraised island barrier-reefs 

 it does not exceed forty feet ; but these regions have been 

 subjected to great denudation. 



In the small island of Santa Anna, which is two and a 

 half miles in length, we have an upraised atoll that dis- 

 plays within the small compass of a height of 470 feet the 

 several stages of its growth. There is, in the first place, 

 the originally submerged volcanic peak ; then the invest- 

 ing soft deposit which, according to Mr. Murray, has the 

 characters of a deep-sea clay ; and over all the ring of 

 coral limestone that cannot far exceed 150 feet in thick- 

 ness. The interior of this upraised atoll is a closed basin 

 containing a fresh-water lake, the bottom of which lies 

 about a hundred feet below the present sea-level : so the 

 island ma\- be roughly compared to a bowl of fresh water 

 floating on the sea. In the vicinity of the locality where 

 the dccp-sea clay was exposed, Lieut. Malan observed a 

 concretionary block of manganese peroxide between one 

 and two cubic feet in size, which, according to Mr. Murray, 

 who examined a typical fragment, is quite similar in cha- 

 racters to the smaller masses obtained in deep-sea sound- 

 ings. The structural history of this island may be briefly 

 summed up. A submarine volcanic peak, having been 

 invested by a deep-sea clay, was brought up by upheaval 

 to the coral zone. An atoll was established on it, and the 

 wholewas subsequently raised to a height of nearly 500 

 feet above the sea. 



Lastly, I come to the large mountainous islands, of 

 which St. Christoval may be taken as the type. This 

 island is more than seventy miles in length, and about 

 4100 feet in height. It is composed in mass of ancient 

 volcanic rocks, which are flanked on their lower slopes 

 by recent calcareous formations. A fawn-coloured crys- 

 talline limestone, containing reef dc^bris, lies directly on 

 the volcanic rock, and is itself overlain by the coral lime- 

 stone. I did not find these calcareous rocks above 500 

 feet above the sea ; so great has been the denudation of 

 this island that these calcareous formations constitute a 

 much thinner crust than that which came under my notice 

 in the smaller and more recent islands. 



Such being the facts, I come now to the four general 

 conclusions, which are as follows : — 



1. T/iaf these upraised reej masses, whether atoll, 

 barrier-reef, or fringing-reef, were formed in a region of 

 elevation. 



This is self-evident. The last upheaval that occurred, 

 of which I found proofs in different parts of the group, 

 was to the extent of about five feet ; but at the present 

 day there arc signs of this movement being still in opera- 

 tion, and, for the purposes of future observation, I have 

 established datum-marks in dift'erent islands. This, there- 

 fore, being a region of elevation, it is apparent that that 

 portion of Mr. Darwin's theory of coral reefs which 

 ascribes the formation of atolls and barrier-reefs to a 

 movement of subsidence cannot be applied to the islands 

 of the Solomon Group, since we here find upraised atolls 

 and barrier-reefs associated with existing reefs of the same 

 description. This conclusion accords with the results 

 obtained by Prof. Semper in the case of the Pelew Islands, 

 and by Prof. A. Agassiz in the case of the Florida 

 reefs. 



2. That such upraised reefs are of moderate thickness, 

 their vertical measurement not exceeding the limit of depth 

 of the reef coral zone. Amongst the numerous islands 

 which I examined I never found one that e.\hibited a 

 greater thickness of coral limestone than 150 feet, or 200 

 feet at the very outside. In fact, so great has been tie 

 denudation of these islands, where, according to my own 



