'638 iV. R. Junner: 



oouipusition 2:ilane UOl well developed. Iddings^ mentions that this 

 type of twinning is often developed by pressure. Extinction angle 

 of the augite varies from 30 deg. to 37 deg. Cleavage parallel to 

 110 perfect. This augite is of interest since in some cases it appears 

 to be almost uniaxial, and the writer was able to determine its 

 sign as positive by the mica plate. A. N. Winchell^ has noted 

 that in a titaniferous pyroxene from Pigeon Point, Minnesota, the 

 optical axial angle is so small that in some cases the mineral 

 appears uniaxial. This seems to be the case in the above-mentioned 

 augite. Inclusions of plagioclase in augite show" that, in part, the 

 augite crystallised out later than the felspar. Augite in the 

 ground mass occurs as eight-sided granules and prisms. The prisms 

 occasionally cross one another, forming stellate aggregates sugges- 

 tive of the rare mode of twinning on (T22).^ Olivine occurs as 

 anhedral crystals of moderate size, now almost entirely replaced by 

 a colourless micaceous mineral with high birefringence, probably 

 talc. Along cracks alteration to green serpentine or chlorite has 

 occurred. Plagioclase is present in long latlis, having a maximum 

 extinction of about 37 deg. The rock may be described as a basaltic 

 dyke. 



2. Dry Creek dyke. Section A25, Dry Creek dyke, near the 

 Plenty River. In hand specimen this is a light-coloured rock, fre- 

 quently iron stained due to oxidation of crystals of pyrite. The 

 minerals present are not determinable in hand specimen. Micro- 

 scopically the texture is holocrystalline and porphyritic. Miner- 

 alogically the rock consists of phenocrysts of orthoclase and quartz 

 in a ground mass of quartz, sericite, bleached biotite, plagioclase, 

 orthoclase, magnetite and kaolin. Brown hydrated iron oxide is 

 fairly abundant. Orthoclase is the chief jjorphyritic constituent. 

 It is present as large, simply twinned crystals, having a maximum 

 extinction angle of about 17 deg. Considerable replacement by 

 quartz and sericite has occurred in many cases. Kaolin is probably 

 a surface alteration of the orthocla.se. Plagioclase liaviiig a maxi- 

 mum extinction angle of 14 deg. from the traces ol' the twin planes, 

 to probably albitc, occurs in consicK'rablc amount in tlic ground 

 mass of the rock. The biotite lias been bleached, antl hydrated 

 iron oxide has been redeposited along cleavage traces, and it is 

 frequently associated with brown piisms of rutile. The consider- 

 able amount of iron oxide tliioughout the section represents the 



1 UldiiiKM. Ro('l< Numerals, p. :i05. 



2 A. N. Wincliell. Notes on .a titaniferous p.M'oxene. Aiuer. Otolosiist, vol. xxxi., 1903. 



3 Iddiii-fS. Op. cit., p. aOf). 



