1 68 A NATURALIST IN THE PACIFIC chap. 



sedimentary basic tuff, presenting layers of coarse and fine 

 materials, and partly palagonitic in composition. It is a little 

 calcareous, and apparently incloses tests of foraminifera. These 

 submarine deposits have evidently been stripped off the basaltic 

 low-lands beneath. It thus becomes evident that the structural 

 features of the Lambasa plains (basaltic rocks overlain by 

 submarine deposits) are preserved to the base of the range. 



These submarine deposits, as exposed in the stream-course, lie 

 beneath agglomerates which repose horizontally upon them ; and 

 from this level up to the bare rocky peak of the mountain, 

 agglomerates and agglomerate-tuffs are alone displayed either as 

 large detached masses or in cliff-faces. 



In the lower part of the mountain the blocks, usually sub- 

 angular, are about a foot across ; but they become smaller as one 

 ascends towards the summit, where they are 3 or 4 inches in 

 diameter. At the top there are extensive exposures in cliff-faces 

 of the agglomerate-tuffs ; and here the finer materials of the matrix 

 include a few rounded pebbles not exceeding half an inch in size. 

 This is a fact of importance in connection with the submarine 

 origin of these formations. 



As regards their composition, the blocks of the agglomerates 

 have not the uniform character we would expect to find in the case 

 of materials directly ejected from a volcanic vent. The most 

 frequent type of rock represented is a grey hypersthene-augite- 

 andesite, having a specific gravity of 272 — 278. It displays small 

 phenocrysts of plagioclase and of rhombic and monoclinic 

 pyroxene, but in other respects it exhibits much variety, not only 

 in the arrangement and average length of the felspar-lathes (-08 

 to 'iS mm.) but in the form of the pyroxene of the groundmass 

 (either granular or prismatic) and in the amount of residual glass, 

 sometimes abundant, sometimes scanty. Two distinct genera 

 (i and S) of the sub-class are therefore represented. 



Other rocks found in these agglomerates contain no rhombic 

 pyroxene, and are referred to genera 13 and 16 of the augite- 

 andesites according to the presence or absence of plagioclase 

 phenocrysts. In the last case we have a dark aphanitic rock 

 (sp. gr. 274), sometimes scoriaceous, where the average length of 

 the felspar-lathes may be as little as '04 mm. One of the blocks 

 was composed of a highly scoriaceous semi-vitreous rock, the 

 cavities being filled with a zeolite. Another was composed of a 

 black porphyritic augite-andesite, showing large crystals of plagio- 

 clase. . . . The matrix of the agglomerate-tuff is formed of sub- 



