Vol. 50.] IGNEOUS ORIGIN ON DAKXMOOE. 345 



The record left by this rock is interesting, inasmuch as it shows 

 that granophyric structure may in some cases, at all events, result 

 from secondary alteration, and does not in these cases proceed from 

 hurried crystallization, and from the imperfect separation of the 

 quartz from the felspar at the time of consolidation. 



All the felspars appear to be orthoclase, and exhibit a single 

 twinning combined with simultaneous extinction. In many of the 

 orthoclases of these rocks, however, there appear to be intergrowths 

 sometimes of microcline and sometimes of plagioclase. The ex- 

 tinctions sometimes point to the former, and sometimes to the latter 

 mineral. 



Trachytes. 



No. 18 1220. Sp. Grr. 275. From place marked 7 on Map 



„ 19 1221. „ 267. „ „ 8 



„ 20 1222. „ 267. „ „ 8 



„ 21 1218. „ 2-66. ,. „ 7 



„ 22 1219. „ 2-65. „ „ 7 



„ 23 1215. „ 2-63. „ „ 6 



„ 24 1226. „ 262. „ „ 9 



The trachytes at 6, 7, and 8 are all exposures of lava-beds 

 in situ ; they occur on the eastern edge of the eastern line of 

 outcrop, and therefore come in below the ash-beds. The spot 

 marked 9 is situated about 50 yards on the north side of the 

 crest of the ridge, and there is a bare patch here with no turf on 

 it, the turf having apparently been removed by the hand of man. 

 On this bare space there are loose blocks of trachyte (No. 24 is a 

 specimen taken from one of them), of ash (No. 10, described 

 on p. 342, is one of these), and of black, carbonaceous, sedimentary 

 rocks. Whether these loose detached blocks represent a bed of 

 coarse agglomerate cropping up in place, or whether they are 

 surface-detritus that accumulated here at some remote period, I 

 cannot say. 



All the above specimens (18-24), with the exception of No. 23, 

 are light-coloured rocks — whitish to whitish-grey on their newly- 

 fractured surface — and have the rough vesicular appearance of 

 trachytes. 



The microscopical examination of thin slices yielded the following- 

 results : — in all except Nos. 19 and 23 the groundmass is composed 

 of a felted mass of microlites of felspar, with some micro-prisms of 

 the same mineral, of somewhat larger size. In No. 19 the ground- 

 mass is cryptocrystalline to micro-granular, and more resembles the 

 groundmass of the felsite previously described. In the great 

 majority of cases, and probably in all, the porphyritic felspars are 

 orthoclases, and exhibit binary twinning and straight extinction. 

 Many of them also show a very irregular striping, which some- 

 times is at right angles to the direction of elongation or to the plane 

 of binary twinning, but generally is parallel to those directions, 

 and indicates an intergrowth of microcline or of some other species 

 of triclinic felspar. The striping has not the regularity, nor does 

 it exhibit the straight planes, of plagioclase ; and in many cases 



