434 3IK. J. PAEKINSOK ON AN INTRUSION OF GRANITE [Aug. 1899, 



That is, about 30'per cent, each of plagioclase and orthoclase and 

 40 per cent, of quartz. The plagioclase will be approximately of 

 the composition AbgAn^. 



III. The Diabase. 



The dial^ase is found at intervals along the cliff eastward from 

 Les Meiuiers. occupying for the most part the higher ground. 

 Near the Semaphore on Eosnez Point several small quarries have 

 been opened in it. Typically it is a black-and-white speckled rock 

 of medium grain, and of a common enough tj'pe. As is usual in 

 rocks of this kind, the degree of coarseness varies considerably from 

 point to point, so that patches, streaks, or veins catch the eye by 

 reason of their conspicuous augites, or because of the greater amount 

 of felspar which they contain. 



The felspar has commonly a greenish tinge. As a typical instance a 

 slide will be described, cut from a specimen collected by Prof. Bonney 

 in 1888, and labelled ' Diorite near junction with coarse granite, 

 La Plaine.' It differs in no respect from my own slides, except in 

 being in a rather fresher condition, for which reason it was chosen 

 for description. 



This rock is an ophitic dolerite, though, as the augite is largely con- 

 verted into hornblende, the term diabase will be used in describing 

 it. The remains of the augite show it to have been of a quite typical 

 kind, colourless or with a pale bluish tinge. It is changing by a 

 gradual assumption of a brownish tint, or frequently by a sharp 

 colour-change, into the usual clove-brown hornblende, which is 

 rather unevenly coloured, and shades off into green so as occasionally 

 to be almost mottled in appearance. The change is usually peri- 

 pheral, the process advancing irregularly inward, but flakes and 

 scales of the brown hornblende also form in the body of the 

 augite-crystal (PI. XXIX, fig. 2). The green hornblende represents 

 a change of the first-formed brown mineral, and there may be a 

 still further loss of this green coloration with the assumption at the 

 same time of a fibrous or actinolitic structure, in some cases rather 

 marked. The felspars are long lath-shaped forms ; they are, judging 

 from the extinction-angles, labradorite or a kindred species. A very 

 few crystals of apatite, and some magnetite and pyrites, complete 

 the list of constituent minerals. 



Occasionally the augite — now almost entirely changed into horn- 

 blende — is present in grains, producing a spotted appearance on the 

 face of a specimen. Its relation to the felspar is, as before, 

 ophitic. A few flakes of a brown, rather fibrous mineral — no 

 doubt a mica — occur in the slide, possibly derived from the horn- 

 blende. In some slides the felspars are almost opaque from the 

 abundance of alteration-products, but there is a tendency for the 

 outer parts to bo comparatively free from these. A zoned structure 

 may then be occasionally observed, but it is not characteristic. 

 Sometimes, as in the small quarry near the Semaphore, specimens 

 may be found markedly amygdaloidal. 



