354 Transactions. — Geology. 



A few years ago I examined tvro rocks in my possession, 

 from the Blnff Peninsula, and found one to be an enstatite 

 diorite. the other a hornblende porphyrite.'' 



Last summer I had an opportunity of ri siting the Bluff for 

 a few hours, and I then collected other specimens of rocks, of 

 which I offer short descriptions. 



HoEXBLZ>~DE DiOBiTE. From Bluff Hill. 



A coarse-grained, granular, hght-gi-ey rock, composed of 

 black hornblende and greyish- white feldspar, the latter usually 

 the more abundant. Specific gravity = 2-S3-5. 



Section. — The feldspar is entirely plagioclase, in plates 

 about OOSoin. square, or in broad laths 0(>46in. long by 

 0-Q22in. to 003in. broad, and well twinned. The hornblende 

 is allotriomorphic, greenish-brown, strongly pleochroic, and 

 partly decomposed to a pale bluish-green, slightly pleochroic, 

 fibrous chlorite. Magnetite is not in great quantity, and is 

 generally associated with the hornblende. There is a Httle 

 pyrites. 



Enstatite Diobite. From Bluff Hill . 



A fine- to medium-grained, granular, dark-grey rock, com- 

 posed of black hornblende and colourless feldspar in nearly 

 equal quantities. Specific gravity = 2-88. 



Section. — The feldspar is entirely plagioclase, in crowded, 

 well-twinned crystals, from 0-Olin. to O-Qoin. in length, and 

 from OOOTin. to 0-OOlin. in breadth. The hornblende is 

 brownish - green, moderately pleochroic, the polarisation 

 colours not brilliant. Augite occurs in qiiite subordinate 

 quantity to the hornblende ; pale-greenish in colour, and form- 

 ing intergrowths with the hornblende : not pleochroic, and with 

 brilliant polarisation colours. Enstatite is in small quantit^-, 

 intergrown with the hornblende ; it is moderately pleochroic, 

 changing from bluish-green to brownish-red ; in ordiuar}' light 

 it is yellow-green. The polarisation colours are not brilliant. 

 Magnetite occurs in crystals and in masses ; it is nearly always 

 associated with the ferro-magnesian minerals. 



HoEXBLENTJE PoBPHTP.iTE. From Green Hills. 



A dark greenish-grey, fine-grained rock, with porphj-ritic 

 crystals of hornblende scattered through it rather abundantly. 

 Specific gravity = 2-901. 



Section. — Ground-mass holocrystalhne, composed of grains 

 and cr\-stals of feldspar and brown hornblende, with some 

 chlorite. The porphyritic minerals are plagioclase and horn- 

 blende. The former is much altered. The hornblende is 

 brown, and partly idiomorphic. There is a httle magnetite 

 and p}-rite3. 



* Jour, of Ro}'. Soc. of N.3. Wales, vol. xxiii., 1889, pp. 13S and 129. 



