Thomas. — On the Bocks of the Kermadec Islands. 313 



A somewhat similar rock, which occurs as a dyke on Meyer 

 Island, an outlying part of Sunday Island, has a darker-grey 

 ground-mass in which augite cannot be seen with the naked eye. 

 The microscope shows a few augite crystals of moderate size, 

 but there is no olivine. The ground-mass is more finely-grained 

 than in the previous rock, and small patches of pale glass can 

 be detected. The felspars include a few sanidines. 



The next rock is one the identification of which is a little 

 uncertain, as it approximates in its characters to the augite- 

 andesites. Mr. Percy Smith states that it is the commonest 

 type of lava on Sunday Island. It is a rock with a black ground- 

 mass of resinous lustre, in which are scattered numerous white 

 felspars reaching 0-2 inch in diameter, giving the rock a con- 

 spicuously speckled appearance. Less conspicuous are a few 

 equally large dark-green augites. The microscope shows that 

 the felspars are nearly all plagioclases ; there are, however, a 

 few sanidines in Carlsbad twins. The ground-mass consists of 

 numerous small slender felspars and less abundant augite, 

 cemented by a glass containing pellucid microliths and dusty 

 grains of magnetite. The proportion between the minute 

 crystals and the glass with microliths varies considerably in the 

 specimens from different parts of Sunday Island. In some 

 varieties the ground-mass consists so largely of glass containing 

 microliths, that this character, combined with the total absence 

 of olivine and the presence of a small proportion of sanidines, 

 entitle the rock to the name of augite-andesite. There appears, 

 therefore, to be a transition amongst the Kermadec lavas, from 

 basalt rich in olivine, through basalt poor or wanting in this 

 mineral, to a characteristic augite-andesite. 



In the yellow volcanic tuff of the cliffs on the north side of 

 Sunday Island, and also along the shore below the cliffs, were 

 found nodular masses of dark-green augite, yellowish olivines, 

 and glassy plagioclase crystals. The augite predominates, and 

 forms crystals reaching an inch in length. Microscopic sections 

 of these nodules show their origin, for here and there between 

 the crystals may be detected a small amount of a volcanic glass 

 with microliths, etc. The nodules are simply aggregations of 

 the large crystals formed in the lavas, and have probably been 

 ejected as bombs during an eruption. 



Glassy rocks. — Eounded bomb-like masses, and irregular 

 fragments of a black rock with pitchy lustre, occur in the recent 

 crater on Sunday Island. There are a few minute crystals 

 visible to the naked eye, but not enough to take away from the 

 lustre of the stone. Under the microscope, the rock is seen to 

 consist chiefly of a brownish glass, finely spotted and clouded 

 with grey. The grey parts are closely charged with rod-like 

 pellucid microliths, and also finer indefinite granules. The 

 brown parts of the glass are not so fully devitrified, and here 



