INTRODUCTION. 



vu 



canic cones. However, the geological evidence at present 

 at our disposal points generally to the great antiquity of the 

 larger islands. The siirnificance of this fact will be subse- 

 quently 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 Shorthand 

 chief. Copper will not improbably be found in association 

 with the serpentine rocks of these islands. 



The smaller islands of volcumc formation group them- 

 selves into two classes : 



(1.) Those which, like Fauro and some of the Florida 

 Islands, are composed partly of modern rocks, such as horn- 

 blende and audte-andesites with their tuffs and acrorlomerates, 

 and partly of ancient and often highly crystalline rocks such 

 as, as I am mformed by Prof. Judd and Mr. T. Davies, 

 quartz-diorites, quartz-porphyries, altered dacites and dole- 

 rites, 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 t3^pe, is pierced by many fuma- 

 roles, and has a crater in the solfatara stage. Savo, though 

 quiescent in the present day, has been in eruption withhi 

 the memory of living men, and was in a state of activity in 

 15C7 when the Spaniards discovered the group. Fumaroles 

 and sulphur-deposits occur in Yella-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 forniations.i Excluding the 



^ Vide my paper ou this subject (Trans. Roy. Soc. Edin. : vol. xxxii., 

 p. 545), and my work on tlie geology of this group. 



