The Solomon Islands and Their Natives

by H. B. Guppy

INTRODUCTION.

The Solomon Islands cover an area 600 miles in length. They include seven or eight large mountainous islands attaining an extreme height, as in the case of Guadalcanar and Bougainville, of from 8,000 to 10,000 feet, and possessing a length varying from 70 to 100 miles, and a breadth varying between 20 and 30 miles. In addition, there are a great number of smaller islands which range in size from those 15 to 20 miles in length to the tiny coral island only half a mile across. The islands fall naturally into two divisions, those mainly or entirely of volcanic formations and those mainly or entirely of recent calcareous formations.

In the first division, St. Christoval may be taken as a type of the large mountainous islands possessing massive profiles, such as Guadalcanar, Malaita, Isabel, etc. St. Christoval, which rises to a height of 4,100 feet above the sea, is composed in the mass of much altered and sometimes highly crystalline volcanic rocks (such as, in their order of frequency, dolerites, diabases, diorites, gabbros, serpentines, and saussuritic felspar-rock) which, as I learn from Mr. T. Davies, have been both formed and altered at considerable depths and indicate great geological age and extensive denudation. Recent calcareous rocks, such as will be subsequently referred to in the description of the second division of islands, flank the lower slopes at the sea-border up to an elevation of 500 feet. Fragments of similar diorites, dolerites, and other dense basic rocks, all much altered and often schistose, have been transported by trees to the coral islets off the coasts of Guadalcanar and afford evidence of the geological structure of that island. Serpentines were obtained by Dr. Hombron in 1838[1] from St. George’s Island, which is “ipso facto” a portion of Isabel. Bougainville and New Georgia are largely of more recent origin, as is indicated by their numerous symmetrical volcanic cones. However, the geological evidence at present at our disposal points generally to the great antiquity of the larger islands. The significance of this fact will be subsequently referred to. There can be little doubt that some of the mountainous islands will be found to yield in quantity the ores of tin and copper. A resident trader, Captain John Macdonald, has discovered arsenical pyrites and stream tin at the head of the Keibeck River in the interior of St. Christoval. A sample of stream tin from the south-east part of Bougainville was given to me by the Shortland chief. Copper will not improbably be found in association with the serpentine rocks of these islands.

[1] “Voyage au Pole Sud et dans L’Océanie,” (D’Urville). Géologie: part ii., p. 211.

The smaller islands of volcanic formation group themselves into two classes:

(1.) Those which, like Fauro and some of the Florida Islands, are composed partly of modern rocks, such as hornblende and augite-andesites with their tuffs and agglomerates, and partly of ancient and often highly crystalline rocks such as, as I am informed by Prof. Judd and Mr. T. Davies, quartz-diorites, quartz-porphyries, altered dacites and dolerites, serpentines, saussuritic felspar-rock, etc.

(2.) Those that are composed entirely or in the main of recently erupted rocks, islands which preserve the volcanic profile, possess craters, and sometimes exhibit signs of latent activity. Eddystone Island, which I examined, is probably typical of the majority of the islands of this class, such as Savo, Murray Island, and many others. It is composed of andesitic lavas of the augite type, is pierced by many fumaroles, and has a crater in the solfatara stage. Savo, though quiescent in the present day, has been in eruption within the memory of living men, and was in a state of activity in 1567 when the Spaniards discovered the group. Fumaroles and sulphur-deposits occur in Vella-la-vella. It may, however, be generally stated that the volcanic forces in these regions are in a quiescent condition at the present day, there being only one vent in active eruption, viz., Mount Bagana in the interior of Bougainville. Many small islands with volcanic profiles show no evidence of a latent activity. Amongst them I may mention those of Bougainville Strait, which are composed of andesitic lavas of the hornblende type.

I now pass to those islands which are composed mainly or entirely of recent calcareous formations.[2] Excluding the innumerable islets that have been formed on the coral reefs at the present sea-level, we come first to those small islands and islets less than 100 feet in height, such as the Three Sisters and Stirling Island, which are composed entirely of coral limestone. In the next place there are islands of larger size and greater height, such as Ugi, which are composed in bulk of partially consolidated bedded deposits containing numerous foraminifera, and possessing the characters of the muds which were found by the “Challenger” Expedition to be at present forming around oceanic volcanic islands in depths probably of from 150 to 500 fathoms. Coral limestones encrust the lower slopes of these islands and do not attain a greater thickness than 150 feet. The next type is to be found in Treasury Island which has a similar structure to that of Ugi, but possesses in its centre an ancient volcanic peak that was once submerged and is now covered over by these recent deposits. Then, there are islands, such as the principal island of the Shortlands, in which the volcanic mass has become an eccentric nucleus, from which line after line of barrier-reef has been advanced based on the soft deposits. These soft deposits contain amongst other organic remains, the shells of pteropods and the tests of foraminifera in great abundance. In such islands I did not find that the coral limestone had a thickness of as much as 100 feet. In this island the upraised reefs are based upon hard foraminiferal limestones. Lastly, we have the upraised atoll of Santa Anna which within the small compass of a height of 470 feet displays the several stages of its growth; first, the originally submerged volcanic peak; then, the investing soft deposit resembling in character a deep-sea clay and considered to have been formed in considerable depths, probably from 1500 to 2000 fathoms; and over all, the ring of coral limestone that cannot far exceed 150 feet in thickness. The islands formed mainly of the soft foraminiferous deposits have long level summits free from peaks. Judging from their profiles, the islands of Ulaua and Ronongo will be found to possess the structure of Ugi and Treasury. The western end of Choiseul has a very significant profile, and I have little doubt from my examination of the lower slopes that this extremity of the island is mainly composed of the recent soft deposits.

[2] Vide my paper on this subject (Trans. Roy. Soc. Edin.: vol. xxxii., p. 545), and my work on the geology of this group.

I now proceed to refer very shortly to the coral reefs[3] of these islands. The three principal classes are to be found in this region; but of these, the fringing and barrier-reefs are more commonly distributed, whilst the atolls are comparatively few in number and of small size. A line of barrier-reefs, probably not much under 60 miles in length and bearing innumerable islets on its surface, fronts the east coasts of the islands of New Georgia at a distance of from one to three miles from the shore. Extensive reefs of the same class, having a broad deep-water channel inside them, lie off the large island of Isabel and off the south-coast of Choiseul. Barrier-reefs, of smaller extent, also skirt the west end of Guadalcanar and the southern end of Bougainville. I have referred particularly to these reefs because at the time that Mr. Darwin wrote his work on “Coral Reefs,” fringing-reefs were alone believed to exist in these islands.