HuTTON. — Geology of the Trelissick Basin. 403 



vesicular palagonite in a calcareous cement. Olivine is rare, 

 but there are occasionally small angular grains of quartz mixed 

 with the palagonite. In the finer upper portion the palagonite 

 fragments are smaller and greener, and there is less calcite. All 

 these rocks break up into irregular cuboidal masses when struck 

 by the hammer. 



Dt/ki's. — Eight dykes are known in the basin, all but oue 

 being clustered round Prebble Hill. Beginning in the south, 

 we find the first (a) on both sides of the Porter Kiver, near Table 

 Hill (pahBOzoic), running east and west ; I could not ascertain its 

 thickness. The second (b) is just south of the fault at the first 

 limestone gorge ; it is nearly vertical, and runs W.N.W. through 

 the greensauds of the Waipara System. Turning eastward, 

 along the south side of Prebble Hill, the next dyke (c) forms the 

 crest of a long spur which runs W.N.W. The fourth (d) is on 

 the ridge forming the watershed of the Broken Kiver ; it 

 also runs W.N.W. The fifth (e) crosses Broken Kiver ; it is 

 nearly vertical, runs N.N.E., and is 15 feet thick. Gonig up 

 the river, the sixth dyke (f) is on the north bauk, and runs 

 N.N.E. The seventh (g) is also on the north bank of the 

 Broken Kiver, but above its junction with the Porter ; it is 12 

 feet thick and runs N.W. The eighth dyke (h) is on the north 

 bank of the Porter, in the marl ; a small fragment, 12 feet long 

 by 8 broad, is all that is exposed : it runs N.W. None of these 

 dykes can be traced higher than the greensands, except h, and 

 this one does not penetrate to the top of the marl. They are all 

 dark bluish-black in colour, and are all composed of a micro- 

 crystalline ground-mass of laths of plagioclase, rounded grains 

 of pale-green augite and magnetite ; but they can be divided 

 into two groups. Dykes a, c, d, e, and h are basalt, with a 

 specific gravity ranging between 2*82 and 2-95, the mean being 

 2-87. They vary from finely granular to crypto-crystalline. 

 They all contain olivine, more or less abundantly, in rounded or 

 broken crystals. This olivine is of two kinds : one is pale green, 

 and shows brilliant colours with polarised light ; the other is 

 colourless, and when revolved between crossed nicols, either 

 merely passes through grey mto black, or else changes from 

 pale bluish-green to pinkish purple. Dykes b, f, and a are 

 augite andesite, with a specific gravity ranging between 2- 59 

 and 2-70, the mean being 2-G4. They contain no olivine, and 

 have a finely granular texture. The position of these dykes, 

 clustered round Prebble Hill and penetrating the green sand- 

 stones only, gives the impression, at first sight, that they may 

 have been connected with a small volcano under Prebble Hill, 

 and that they were formed before the marl and limestone were 

 deposited. But there are no traces of contemporaneous volcanic 

 action in the green sandstones, nor in the marl, while only one 

 of the dykes has penetrated so far upward as the lower part of 



