3 i4 A NATURALIST IN THE PACIFIC chap. 



The Volcanic Agglomerates 



In this place my remarks will be chiefly confined to a summary 

 of some of the leading features of the"se formations. The 

 agglomerates, which pass by all gradations through the tuff- 

 agglomerates into the submarine tuffs, rank amongst the most 

 prevalent and the most conspicuous of the rocks exposed at the 

 surface in this island. Their lithological characters vary according 

 to the type of the massive rocks of the district. Thus in the 

 Ndrandramea district the blocks are composed of the prevailing 

 acid andesites. In the Koro-mbasanga district they are formed of 

 hypersthene-augite-andesites. In the Korotini and Va-lili ranges 

 they are composed of olivine basalts and basaltic andesites. The 

 agglomerates derived from basaltic rocks and basic andesites are by 

 far the most frequent, and it is to them that the following general 

 observations apply. 



The basic agglomerates and tuff-agglomerates are found almost 

 everywhere and at all elevations up to 2,500 feet above the sea and 

 over. They compose the inland cliffs and the long lines of 

 precipitous declivities that give character to the valleys and gorges 

 of the mountainous interior. The blocks are often scoriaceous 

 and semi-vitreous, but the characters of the rocks will be found 

 described on page 316. They are generally sub-angular and vary in 

 size from a few inches to one or two feet ; and, though sometimes 

 heaped together in confusion, they will generally be found in the 

 case of any extensive exposure to be ruddy sorted according to 

 size, or to present a rude horizontal arrangement. 



The matrix varies much in amount, being sometimes barely 

 appreciable and at other times so abundant that the deposit may 

 be termed a tuff-agglomerate. Typically it has the character of the 

 palagonite-tuffs of mixed composition described on page 326, being 

 made up of fragments of palagonitised vacuolar basic glass, por- 

 tions of crystals of plagioclase and augite, with the debris of the 

 basic semi-vitreous and hemi-crystalline rocks forming the blocks. 

 When it is scanty it contains neither carbonate of lime nor organic 

 remains ; but in the tuff-agglomerates it may be calcareous and 

 may inclose tests of foraminifera and molluscan shells. 



From the circumstance that the basic agglomerates overlie 

 submarine sedimentary tuffs and clays almost everywhere, their 

 submarine origin could alone be safely postulated. There are one 

 or two localities that throw especial light on the conditions under 



