Thomas. — On the Rocks of the Kermadec Islands. 311 



angite are present iu small quantity. The ground-mass is partly 

 micro-felsitic, and consists of dark bands of micro-felsite •which 

 alternate with bands, clearer bands, in which the micro-felsitic 

 matter passes into a finely micro -crystalline aggregate with red 

 granules. 



The rock which forms the island of Motutaiko is a bluish- 

 grey laminated lava, with semi- vitreous glaze along the surface 

 of the joints. The thin parallel laminae of which the rock is com- 

 posed are often very distinct, and are alternately more or less 

 glassy, but in other parts the laminae blend together, as the glass 

 becomes more fully devitrified. The macroscopic crystals are 

 not numerous, and are chiefly sanidines ; plagioclase is, however, 

 also present, as well as augite, magnetite, and apatite. The 

 ground-mass is chiefly a pale-glass, crowded with slender 

 microliths and black granules, but it is marked here and 

 there by dark-brownish patches and streaks of indefinitely 

 granular or micro-felsitic matter, giving a banded character to 

 the rock. 



It is unnecessary to mention here the pumice, which occurs in 

 such vast quantities in the district. A nearly pure obsidian was 

 obtained by Mr. Cussen on Ngauruhoe. It has a perfect con- 

 choiclal fracture, and no macroscopic crystals. In thin flakes it 

 shows a greenish-smoky colour and bands of a darker shade. 

 The general mass of the rock is quite free from microliths, but 

 the bands derive their darker appearance from the presence of 

 multitudes of parallel microliths. 



Art. XLI. — Notes on the Bocks of the Kermadec Islands. 



By A. P. W. Thomas, M.A., F.L.S., Professor of Natural Science 

 in the Auckland University College. 



[Bead before the Auckland Institute, lAth November, 1887.] 



The following notes are descriptive of the various rocks collected 

 by Mr. Percy Smith during the recent visit of the Stella to 

 the Kermadec Islands. Most of the specimens were obtained 

 from Sunday Island, the largest of the group. 



The rocks, with one curious exception, are unmistakably of 

 volcanic origin, and consist of fragmental materials in the form 

 of scoriae and fragments from the tuffs, and of lavas from the 

 streams which alternate with the tuffs, or from the dykes which 

 intersect the islands, and bind the loose beds and lava streams 

 together. 



