Rocks of Noyang. 35 



former become poor in mica or amphibole, and at the same 

 time acquire a more porphyrite character by the appearance 

 of isolated crystals of felspar or quartz, or of both. 



Great portions of Mount Elizabeth, in fact masses which 

 are mountains, are, it seems, entirely composed of such rocks 

 as those I am now considering, and in situ they have an 

 extraordinary resemblance to their close analogues, the 

 quartz-porphyries. Kocks of this class are in contact with 

 the sediments over as large an area as are the crystalline- 

 granular quartz-diorites. 



I have included the quartz-granophyrites in this section,, 

 for the reason that I have as yet found such rocks only as 

 abnormal parts of the quartz-mica-porphyrites. At the 

 Haunted Stream, for instance, the great mass of such rocks 

 at its northern contact with the sediments, has the structure 

 of a granophyrite. 



Some of the larger dykes which proceed from the quartz- 

 porphyrite masses across the crystalline-granular rocks also 

 show this structure. 



As I have already said, I have not found the granophyrite 

 structure largely developed at Noyang; but it does not follow 

 that it is not to be met with more frequently than I have 

 found it. The area of the quartz-porphyrite rocks is so large, 

 that my examination, which has been specially directed to 

 the area surveyed, has only included it in part. 



The results of microscopic and chemical analysis of these 

 rocks are as follow. The first example which I shall describe 

 was collected near the north-western contact in Navigation 

 Greek : — 



Under the microscope it shows a micro-cry sta] line granular 

 ground-mass, wholly composed of felspar and quartz, to- 

 gether with very numerous bladed microliths of a light green 

 colour, and these occur in both felspars and quartz. So far 

 as I could make out from an examination of these minute 

 minerals, I believe them to have been amphibole, but now 

 almost, if not quite, converted into some form of chlorite. 



In this ground-mass, and forming part of it, certain of its 

 constituents are porphyritic : — 



(1.) Small twinned crystals, or clusters of crystals, of fel- 

 spars, in which I found optical measurements in the zone 

 OP — oo P go min. = 5 Q , and max. = 15 9 15', and certain 

 simple sections, in which the angle formed by the plane of 

 vibration was 14°. The composition of these twins was 

 according to (a) the Albite law; (b) the Carlsbad law; (c) 



