xv NAILOTHA 215 



felspar-lathes with fine decomposing pyroxenes are shown. The 

 lathes give extinctions of two or three degrees and average '04 mm. 

 in length. In its somewhat scoriaceous nature, in the absence of 

 phenocrysts, and in its less altered condition, this rock differs from 

 those exposed higher up the ravine ; but it is evidently to be 

 referred to the same acid type. At a height of 300 feet a light 

 grey highly altered oligoclase-trachyte (sp. gr. 2*43) is exhibited. 

 It contained originally some phenocrysts of felspar and pyroxene, 

 which, however, have been more or less replaced by calcite, quartz, 

 and other materials ; whilst the groundmass, originally hemi- 

 crystalline but now blurred by the deposition of silica, shows 

 felspar-lathes in process of development. 



A little farther up the gorge there is displayed another highly 

 altered rock with a siliceous appearance, such as has been noticed 

 above as forming blocks in the stream near the foot of the moun- 

 tain. It is an oligoclase-trachyte impregnated with crystalline 

 silica and exhibiting a singular prismatic structure, the small 

 columns or prisms being only 3 or 4 inches in diameter. Between 

 400 and 500 feet occurs a light-coloured compact rock sparkling 

 with pyrites and also displaying a columnar structure, the columns 

 being between 4 and 6 inches across. It looks like a limestone 

 and effervesces freely with an acid ; but it is in fact a highly altered 

 oligoclase-trachyte (sp. gr. 2-5) of the propylite type. It seems 

 originally to have inclosed a few phenocrysts of oligoclase, sanidine, 

 and pyroxene, and here and there a stout felspar-lathe is to be 

 noticed in the groundmass giving straight extinction. The whole 

 texture of the rock, however, is more or less impregnated with 

 secondary quartz, calcite, chlorite, viridite, pyrites, &c. Farther up 

 the ravine, between 500 and 600 feet in elevation, is displayed a 

 remarkable quartz-porphyry which exhibits opaque porphyritic 

 crystals of felspar as well as rounded crystals of quartz in a grey 

 compact base. It has been subjected to considerable alteration 

 and will be found described on page 310. 



At 600 feet large slabs of the ordinary sedimentary clay-tuffs, 

 containing but little lime and showing no organic remains to the 

 eye, lay about on the more level slopes, and evidently formed a 

 surface deposit incrusting the altered massive rocks. If my ascent 

 had lain up the mountain-side away from the stream-courses, these 

 sedimentary rocks would alone have been observed. In the gorge, 

 however, is exposed to view the deeper-seated rocks that make up 

 the mountain's mass. We have here a thickness of about 400 feet 

 of altered acid rocks. All are doubtless intrusive, and were sub- 



