BY W. N. BENSON. 695 



Related to these rocks, are two other groups of porphyries. The 

 first group may be termed the grey porphyries, being less dark than 

 the last group. They are very frequent about the head of Munro's 

 Creek, and are characterised by the more coarsely crystalline 

 nature of the felsitic base. The felspar-phenocrysts are generally 

 smaller and often fresh. The hornblende is usually much smaller, 

 and is considerably decomposed to chlorite, or to chlorite, biotite, 

 and calcite [N.T., 82], and the secondary material is distributed 

 about the rock. In one [N.T., 33], hornblende forms long, faintly 

 coloured prisms of a brownish-pink tint, and decomposes to red- 

 brown chlorite. There is often apatite noticeably present, and 

 quartz is not infrequent in the base, and occurs occasionally 

 as xenocrysts [N.T., 82]. The ground-mass is usually a mosaic 

 of irregularly granular material, but sometimes [N.T., 319], it is 

 pilotaxitic. This last rock is remarkable in containing a little 

 pyrite. Its porphyritic character is not pronounced, and still less 

 so is that of N.T., 387, which has quite a schistose appearance in 

 hand-specimen. Orthoclase is present in these rocks, but only in 

 small amount. 



A second variation is afforded by the granophyric porphyries, in 

 which there is a considerable amount of micrographic structure. 

 A regular sequence can be traced, from rocks in which spherulitic 

 structure is just suggested in the base mosaic [N.T., 42, 99], to those 

 in which it becomes well marked in the base [N.T., 142, 516] ; and. 

 finally, to those rocks in which the whole base is granophyric, and 

 set with seriate* phenocrysts of plagioclase. In this rock, biotite 

 replaces hornblende. 



The quartz-porphyries have also their fine- and coarse-grained 

 varieties. Several, very fine-grained examples occur on the water- 

 shed between Duncan's and Munro's Creeks. They are grey or 

 creamy-pink in colour, and consist of more or less idiomorphic 

 quartz-grains often strained, in a micro-felsitic base, which some- 



* A useful term suggested by Iddings. 



