324 MR. ALFRED HARKER OK THE [May 1896, 



It is probable that the needles of apatite seen in most of the 

 slices have been in part derived intact from destroyed gabbro-frag- 

 ments, but this is not susceptible of decisive proof. Similar needles 

 occur in the normal granophyres as well as in the gabbros of the 

 district, and their rather capricious distribution in both rocks 

 renders unsafe any argument founded on the relative amounts of 

 the mineral in different slides. 



It is the augite that affords the most conclusive proof of the 

 extraneous origin of the xenocrysts, and this is due to the charac- 

 teristic basal striation of the gabbro-augite, a feature not found in 

 the augite of the normal granophyres. In the recognizable enclosed 

 fragments of gabbro (6704) * the augite shows no change except a 

 conversion to brownish-green, rather fibrous hornblende at the edge 

 of the crystal, a transformation very common in the ordinary gabbros 

 of the district (PI. XIII. fig. 1\ In the isolated xenocrysts the 

 conversion to hornblende is usually far advanced, and in these rocks 

 in general this mineral predominates over augite. It is yellowish to 

 brownish -green or sometimes greenish-brown in colour, and of com- 

 pact (as contrasted with fibrous) structure. Very often there is a core 

 of unchanged augite with the basal striation that indicates its deriva- 

 tion from gabbro, and the traces of this structure are sometimes seen — 

 even when the conversion to hornblende has been complete (PI. XIII. 

 fig. 3). Failing this evidence, the derivation of the hornblende can 

 often be inferred from the irregular shape of its crystals, or from its 

 enclosing abundant shapeless grains of magnetite. On the other 

 hand, there is usually some hornblende presenting the crystal outlines 

 proper to that mineral, and this must certainly have crystallized out 

 from the modified granophyre-magma (PI. XIII. figs. 4. 5). In some 

 slides it is very plentiful. It does not differ materially in colour and 

 pleochroism from the pseudomorphic hornblende. It may be re- 

 marked that when the latter encloses a core of unchanged augite the 

 two minerals have the usual cry stall ographic relation, the b and c axes 

 being common to both: in a clinopinacoidal section the extinction- 

 angle of the augite is 39°, and of the hornblende 18°, on the same 

 side of the vertical (2674). In addition to the augite plainly derived 

 from gabbro, several of the slides contain rather rounded grains of 

 augite showing neither basal striation nor partial conversion to horn- 

 blende. Unless these be relics of vanished xenoliths of basalt, they 

 are probablv to be regarded as having crystallized out directly from 

 the modified granophyre-magma (PI. XIII. fig. 1). This would not 

 be remarkable, for augite is widely distributed in the normal grano- 

 phyres of the district, where it often occurs side by side with original 

 hornblende. Since, however, non-striated augite is found in many 

 of the gabbros, the absence of this structure cannot in itself be 

 regarded as conclusive. 



At Barnavave the xenocrysts of diallage are described by Prof. 

 Sollas as showing three different lines of alteration, the characteristic 

 products being respectively granular augite, biotite, and green horn- 



1 The numbprs between parentheses are those of the slides in the collections 

 of the Geological Survey. 



