16 



all tlie microcline was assumed to be perthitic and to have 

 the composition found by Gartrell for the phenocrysts. 



The mineral constitution and the chemical composition 

 alike indicate that the rock belongs to the Adamellite division 

 of the granite family. 



(2) QUARTZ MICA DIORITE. 



A number of specimens of this rock, chosen to illustrate 

 the variation in characters, have been sectioned. No. 1 repre- 

 sents the quartz maca diorite proper, without any large 

 crystals. It is a dense dark-blue rock, and under the 

 microscope is seen to be holocrystalline, of fine variable grain 

 size, and hypidiomorphic granular. A Rosiwal measurement 

 gave the following percentage mineral constitution by 

 weight : — 



Quartz 36*1 



Plagiolcase ... ... ... ... 41*7 



■ Biotite 20*9 



Ilmenite ... ... ... ... 1*3 



One or two sections of potash felspar were also noticed, 

 and a little sphene and apatite occur. 



Plagioclase is in short idiomorphic prisms, averaging 

 about '4 mm., and is slightly dusted with alteration products. 

 Some sections are zoned, and the composition appears to range 

 from about Ab52 to Ab^g. 



Biotite is quite fresh, with a strong, rather greenish- 

 brown colour; it inclines to idiomorphism at times. Included 

 in it there are commonly very tiny granules of sphene bunched 

 together or in strings, and often clustering round the edges 

 of the mica. The biotite appears to have crystallized partly 

 before and partly after the felspar. 



Quartz is granular and interstitial, wrapping round and 

 sometimes including the felspar and other minerals. It is 

 notably free from cracks and inclusions, in contrast to the 

 opalescent quiartz of the porphyritic granite. 



Ilmenite is not very abundant. It is sometimes altered 

 to secondary sphene. How far the sphene in the rock is 

 primary is uncertain ; a little does certainly occur in sub- 

 idiomorphic sections, but much of the minutely granular 

 material associated with the biotite is probably secondary. 



Microlitic needles of apatite are fairly abundant, but 

 strangely enough very little of it is included in the biotite. 

 A certain amount of strain is indicated in the rock. The 

 absence of zircon in the biotite is noteworthy, likewise the 

 virtual absence of the potash felspar from the rock. 



