252 PROCEEDINGS OF SECTION C. 



The ground-mass is abundant, pale-brown when not devitrified, 

 with rather large grains of magnetite scattered through it, and 

 occasionally felspar microlites which shew no fluxion structure 

 except in the case of a specimen from the north side of Coromandel 

 Harbour. 



The felspars are plagioclase, zoned, but not much twinned. In 

 the Tapu Greek rock some are not twinned at all, and others are 

 binary twins which might easily pass for sanadine. But one 

 of these binary twins gave extinction angles of 10° 30' and 14° 30' 

 on either side of the composition plane, while none shewed 

 simultaneous extinction ; they cannot, therefore, belong to the 

 Monoclinic system, but may perhaps be oligoclase. The felspars 

 are generally clear, but decomposed along cracks in the Tapu rock, 

 and very much decomposed in those from Karaka and Waiotahi 

 Creeks. 



Hornblende. — In the Tapu rock the hornblendes are mostly 

 fresh, but with a narrow black border. Prismatic cleavage is not 

 well marked. They are strongly pleochroic, changing from light 

 to dark yellowish brown. The angle c : y up to 19° 30'. In the 

 rocks from the Thames and Coromandel the hornblendes are 

 entirely altered to a colourless aggregate with black borders, but 

 can be recognised as six-sided prisms, shewing the faces 110 and 010. 



Augite, unaltered, occurs only in the specimen from the north 

 side of Coromandel Harbour. It is clear, pale greenish-yellow, 

 with brilliant polarization colours, not pleochroic, and often 

 twinned. There are no apatite inclusions. 



Chlorite is of two kinds. The first is of a pale blue-green, fibrous 

 in places, feebly pleochroic, and with low polarization colours ; it 

 is found in some of the hornblendes more or less filling them. 

 The second kind is bluish-green, not pleochroic and nearly isotropic ; 

 it is found in small qviantity in square prisms as pseudomorphs 

 after augite. 



Bastite occurs in the Karaka Creek rock, but in quantity quite 

 subordinate to the hornblende. It is distinctly cleaved and 

 pleochroic, and pale yellow-brown ; ^ and y bright blue-green. 



Calcite is to be recognised in the decomposed felspars and horn- 

 blendes, as well as in the ground-mass of the Thames rocks. 



Gypsum (?). — In some of the decomposed hornblendes the 

 colourless aggregate which fills them shews, in places, bright 

 polarization colours. This may be due to talc, or more probably 

 to gypsum, as Mr. Skey has ascertained that this mineral occurs 

 in some of the rocks ; but the optical characters are uncertain. 



Quartz (secondary) is also present in the ground-mass of those 

 rocks which contain calcite. 



Titaniferous Magnetite. — Abundant both as an original con- 

 stituent and secondary. In the Waiotahi rock some of it is 

 changed into leucoxene, and in the Tapu rock into limonite. 



