312 Transactions. — Geology. 



The lavas are of a basic character, and consist of basalt and 

 augite-andesite, together with glassy varieties of basic rocks. 



Basalt. — One of the most characteristic types occurs in a 

 stream on the east coast of Sunday Island. It is a dark porous 

 rock in which the larger crystals can just be seen with the naked 

 eye. In microscopic sections these are seen to consist of 

 numerous plagioclases and olivines, with yellowish-green augites. 

 The plagioclases are in slender prisms, and seldom show any 

 inclusions. The ground-mass of the rock is very finely crystal- 

 line, and is composed of felspar, crippled augite grains and 

 crystals, and minutely octahedral magnetite. No amorphous 

 matter can be distinguished between the elements of the base. 

 The magnetite is so abundant as to render the ground-mass 

 very dark, and the crystals are frequently grouped along lines 

 at right angles to one another, so as to form the most beautiful 

 branched figures. 



The recent lava from the Cascade, in Macaulay Island, differs 

 considerably in its macroscopic crystals. These consist chiefly of 

 plagioclases reaching 0-25 inch in diameter, which, in striking 

 contrast to those in the former variety, are in short stout crystals, 

 showing a zonal structure, from the abundance of the inclusions 

 of glassy matter. These inclusions are often accurately rect- 

 angular, their longer sides being parallel to the neighbouring face 

 of the crystal. Even the minutest inclusions, which occur in 

 such abundance as to appear under a low power like dusky 

 clouds, generally show this rectangular form. The only other 

 crystals which reach any considerable size are a small number of 

 olivines, containing magnetites. The ground-mass is much like 

 that of the rock first described, the magnetite, however, though 

 equally abundant, does not form the branched groups of crystals. 



The scoria? from the recent crater on Macaulay Island are 

 also basaltic, differing chiefly from the lava at the Cascade in 

 containing a good many augites of the first order of separation. 

 Another basalt which occurs on the east coast of Sunday Island is 

 rich in olivine crystals, and has a ground-mass of more coarsely 

 crystalline granular structure. The large macroscopic plagio- 

 clases have numerous inclusions of brown glass, often arranged 

 in long streaks alternating with the lamella? of the crystals. 



The common grey lava from the north coast of Sunday 

 Island has a rather light-grey and finely crystalline ground-mass, 

 in whch small augites and felspars can be seen with the naked 

 eye. The felspars seem to be all plagioclases, and are fairly rich 

 in glass inclusions. The rock contains no olivine, but must be 

 considered as a basalt, as the ground-mass is of a distinctly 

 basaltic type, consisting of a crystalline aggregate of plagioclase, 

 augite, and not very abundant magnetite, the individual ele- 

 ments being unusually distinct. No glass can be recognised 

 with certainty in the ground-mass. 



