364 Watts and Newton — Rocks from Solomon Islands. 



(4) Only one sample of ftypersthene-andesite has reached me ; it 

 is from a rounded pebble found 40 feet above high-water level 

 on a small island on the reef in the Gizo group (F. 244). It shows 

 glassy felspars in a grey andesitic matrix. The section exhibits 

 zoned plagioclase felspars, which belong to the more acid division of 

 the labradorites, a few green hornblendes, small augites, and fairly 

 abundant crystals of hypersthene, imbedded in a brown-tinted glass 

 containing a few felspar microlites and black trichites in such plenty 

 as to give the glass its brown colour. In the British Museum 

 collection I have noticed similar hypersthene in the andesites from 

 the Shortland Islands, and Prof. Judd has noted its occurrence on 

 Simbo or Narovo Island. 1 



(5) The single example of hornblende-andesite comes from the 

 summit of Evorai Mountain, on the north coast of New Georgia 

 (F. 294), and is much decomposed. It is of a dirty grey colour, 

 and has scattered through it large porphyritic crystals of hornblende, 

 some of them being half an inch long. With a lens, felspar crystals 

 and smaller ferro-magnesian minerals can be seen. The fine 

 collection in the British Museum contains a large number of rocks of 

 similar microscopic character, and in this collection every gradation 

 from hornblende to augite-andesite can be studied. A specimen in 

 Prof. Judd's collection from the north part of Piedu Island is also like 

 this rock in macroscopic aspect. Plagioclase felspar, a labradorite, 

 with zoning and multiple twinning, is abundant as a porphyritic con- 

 stituent. The hornblende is deep brown in colour, and usually has 

 an opaque black border, which sometimes extends into the crystal 

 or even replaces the whole of it. Iron-ore granules and a few 

 crystals of augite are present, and there are also pseudomorphic 

 patches of chlorite and magnetite which appear to replace the latter 

 mineral. The ground-mass is of minute felspar crystals, matted 

 together in a clear glass, which also contains iron-ore dust and 

 serpentinous replacements of a ferro-magnesian mineral. 



(6) Aagitic trachyte. — A remarkable rock, which probably belongs 

 to this class, occurs on the slopes of Kutelike (F. 2S8). It is not 

 like the other rocks, nor, indeed, is it quite like any one with which 

 I am acquainted. It is light grey and granular in aspect, traversed 

 by innumerable minute, joint fissures, and exhibiting on the broken 

 surface crystals of black augite and occasional felspars imbedded in 

 a white matrix. Under the microscope the bulk of the rock is seen 

 to be made up of small crystals of felspar, which are rhomboidal in 

 section, and are almost entirely devoid of twinning, except now and 

 then on the carlsbad plan. They are beautifully zoned, the outside 

 extinguishing at a lower angle than the interior. The refractive 

 index, where it can be determined, appears to be higher than that of 

 canada balsam. On placing the powdered rock in a diffusion column 

 of methylene iodide, about half the felspar floats in liquid with a 

 density of 2-57, proving that a good deal of the felspar is orthoclase. 

 Another portion floats at 2-62. This seems to indicate that orthoclase 

 and albite are present in about equal proportion, and it is quite 



1 " The Solomon Islands, their Geology," etc., p. 44, footnote. 





