Geology. 277 



of the eruption of a mass of basalt along the same line as that along 

 which an eruption of trachyte had taken place at an earlier date, 

 and the inclusion of portions of the older lava in the newer. The 

 trachyte at this point is made up of a felt of small lath-shaped 

 felspar crystals, giving straight extinctions and sometimes showing 

 traces of a radial arrangement in groups. There are no porphyritic 

 constituents and there are scattered grains of magnetite. 



The basalt (marked P in section B, Pigs. 2 and 5) varies much 

 in character in difierent parts of the mass. Por the most part it 

 is a fine-grained rock, consisting of small lath-shaped crystals of 

 felspar, a little granular augite, and much magnetite. Plow 

 structure is shown in the ari-angement of the felsj)ar crystals ; 

 there are no porphyritic crystals. Where this rock forms a narrow 

 tongue penetratiug the trachyte it becomes more glassy. In one 

 place the mass of fine-grained basalt is traversed by a vein or 

 dyke of a basalt with large porphyritic crystals of pinkish-brown 

 augite, often showing the characteristic octagonal sections, large 

 fi'agments of olivine sometimes showing traces of crystal forms 

 and altered into viridite along cracks only, and finally large lath- 

 shaped crystals of felspars with traces of zoning ; in one or 

 two instances felspars are included in the augite crystals. The 

 ground-mass consists of felspar microliths, small augites, and much 

 magnetite. This coarser rock was probabl}^ injected into a fissure 

 from the deeper portion of the mass. 



South of the fault X-X the limestone B rests on the upper 

 end of a great mass of basalt (P), which can be traced down nearly 

 to the sea-level. At its upper end it penetrates the limestone, 

 and is repeated twice owing to slight faulting parallel to the main 

 fault X-X. 



Wherever the talus is wanting it can be seen that the yellow 

 limestone (B) is overlaid by a bed of glassy basalt (G), varying 

 greatly in thickness and attaining its greatest develoj)ment towards 

 the southern end of the bay (nearly above the letters jST.P.-S.AV. in 

 section A, Pig. 2). At this jDoint it forms the lower portion of 

 a lofty overhanging cliff, which, from the red staining of the lime- 

 stone forming its summit and the colour of the volcanic ash 

 beneath, is called by the people the " J^atoe merah" or the "red 

 I'ock." To the north of this point the basalt bed can be seen 

 at intervals only, and to the south it is interrupted (see Pig. 2) 

 by the fault X-X and is finally cut out by the slip Y-Y, the 

 Miocene Orbitoidal limestones (C) resting on its end. 



The basalt of this bed is a somewhat glassy rock, the ground-mass 

 of which is full of microliths of felspar and granules of magnetite ; 

 there is a considerable quantity of olivine, which except in a few 

 cases is entirely altered into serpentinous material. iS^umerous 

 founded vesicles more or less completely filled with a bottle-green 

 substance are present. In the higher part of the bed this rock 

 has undergone further alteration, the whole of the olivine being 

 replaced by serpentine and the green material filling the vesicles 



