Watts and Newton — Rocks from Solomon Islands. 363 



to pale-yellowish green. The olivine is frequently idioraorphic ami 

 tends to occur in single individuals rather than in groups ; and 

 although usually quite fresh, it has often a brown border, and is 

 sometimes converted entirely into the substance known as iddingsite. 

 Large grains of iron-ore are generally present, but phenocrysts of 

 felspar are small and scarce, and they tend to graduate down to the 

 smaller crystals of the ground-mass; the extinctions appear to 

 indicate that this felspar is labradorite. The ground-mass consists 

 of augite granules, iron-ore dust, and felspars in the form of stumpy 

 prisms, laths, or needles, all of which are striated, with a very little 

 brown mica. The felspars usually show flow-structure, and a 

 little interstitial glass can be made out. One variety of this rock 

 (F. 289) contains a rather larger number of labradorite phenocrysts, 

 which are, however, not large in size. 



(2) Porphyritic olivine -basalt with abundant phenocrysts of felspar 

 occurs at the summit of Dubatina Mountain (F. 295) and at the 

 summit of Kutelike Mountain (F. 283), portions of ranges which 

 run north and south respectively from the Karu Mahimba 

 Kange. The rocks are rather darker in colour than the last type, 

 and they show needle-like, twinned crystals of felspar on their 

 broken surface, with occasional dark crystals of augite. The 

 weather appears to penetrate deeply into them, and one of the 

 specimens is coated with a thick brown crust. The principal pheno- 

 crysts consist of a felspar, apparently labradorite, with a high 

 refractive index ; usually it has a clear border which extinguishes at 

 a lower angle than the core, and into this the twin striation penetrates. 

 The augite is like that already described, but it is beautifully zoned, 

 and the olivine which is not very abundant, is in small grains more 

 or less replaced by serpentine. The ground-mass consists of stumpy 

 felspars, augite granules, and magnetite dust, and it does not appear 

 to contain any unaltered glass. In one specimen of this rock 

 (F. 295) the felspar is commonly in cruciform twins, and the crystals 

 possess cores or belts of minute inclusions with a border of clear 

 felspar. 



(3) The augite-andesites require little notice, as most of them are 

 identical with those described in Dr. Guppy's book. One specimen 

 (F. 286) was collected near the sea-level, and one (F. 291) at the 

 summit (672 feet) of Marovo Island, which is practically the south- 

 eastward extension of New Georgia. The rocks are light grey in 

 colour, with an irregular fracture, and a rather horny surface which 

 shows phenocrysts of felspar. No olivine is present, and augite is 

 by no means so common as in the basalts. The principal phenocrysts 

 are of plagioclase felspar, either as carlsbad or albite twins, the latter 

 giving such extinctions as to show that the felspar is somewhere 

 between andesine and labradorite. The interior of the felspars often 

 exhibits a moiree structure, which is not observable in the outer 

 shell. The ground-mass is made of elongated felspar microlites 

 giving extinctions proving that they belong to the oligoclase-andesine 

 series, with augite granules and a small quantity of iron-ore dust set 

 in a brown glass, which passes locally into bright orange patches 

 with a weak aggregate polarization, apparently a form of palagonite. 



