A. Harl-er — Rocks from the Tonga Islands. 257 



a rather fibrous rhombic pyroxene. Some of the felspars are 

 andesine, but they are not all of one variety. With these minerals 

 occur many fragments of yellowish glass, often with very irregular 

 shape and concave boundaries, such as would arise from the breaking 

 up of pumice. 



The coarser accumulations would rather be termed agglomerates. 

 They have a rough and often porous texture, and are largely com- 

 posed of bomb-like ejectamenta, a quarter or half an inch in diameter, 

 besides pieces of vesicular and pumiceous lava. The interspaces are 

 often partly vacant, with a dusty lining. These agglomeratic rocks 

 are grey or yellowish, but become red or brown by weathering. 



Mr. Lister's collection from Falcon Island includes a number of 

 specimens from the ejected blocks of lava scattered over the island. 

 These are more or less vesicular rocks, showing numerous little 

 glassy felspars, one-tenth to one-fifth of an inch long, imbedded in 

 a dark grey ground. Some, perhaps rather weathered, show a 

 lighter grey ; while others, more glassy, are black with a silky 

 lustre. The vesicles usually vary from mere pores to cavities an 

 inch or more in length ; but in some examples they are drawn out 

 into narrow pipes more than six inches long. The more scoriaceous 

 and cellular varieties are free from porphyritic crystals, and the 

 perfectly glassy pumice is pure white. 



These lavas appear to be basic augite-andesites, neither olivine 

 nor rhombic pyroxene being detected. One of the sliced specimens 

 gave a specific gravity 2'436, but this is evidently too low, and 

 indeed the specimen contains numerous microscopic vacuities ; a 

 more compact example gave 2-708, which seems to indicate a 

 decidedly basic composition. 



In the slices [1264-1266] the porphyritic felspars seem, from 

 their extinction-angles, to be bytownite. They have Carlsbad and 

 albite-twinning, and some of the larger ones show pericline-lamellse 

 in addition. The crystals are well bounded, but often grouped in 

 clusters so as to interfere with their perfect development. The 

 crystals are clear, but contain glass-cavities, usually with zonal 

 disposition, and some entangled portions of the ground-mass. The 

 augite, pale yellow in section, is not very abundant. Some of the 

 larger crystals are so associated with the porphyritic felspars as to 

 prove that they belong to an early phase of consolidation, but the 

 bulk of the mineral occurs in ill-shaped idiomorphic crystals 

 scattered thi'ough the ground. The ground-mass consists of numerous 

 lath-shaped microlites of felspar imbedded with a more or less 

 fluxional arrangement in an isotropic glass. The proportion of 

 glassy base varies considerably, as might be inferred from the 

 appearance in hand-specimens. A very characteristic feature is 

 the occurrence in the mass of well-defined irregular, or usually 

 round, patches of lighter colour and containing less isotropic base 

 tlian the surrounding mass. These seem to be portions of lava 

 which while partly consolidated became involved in a more fluid 

 magma [1264]. 



With the exception of the Falcon Island rocks, all those examined 



DECADE III. VOL. VIII. NO. YI. 17 



