118 DR. J. W. EVANS ON THE ROCKS OE THE [Feb. 1906, 



amount of alkalies present is nearly the same in both cases, there 

 being '111 molecule in the actual analysis and *104 in that calcu- 

 lated from the microscopic measurements ; but, whereas there are 

 •054 molecule of potash and -057 of soda in the former, the 

 calculated amounts were *087 and '014. It is evident that the 

 orthoelase and microcline contained a considerable amount of soda 

 — either in microperthitic inclusions, or simply replacing potash in 

 the orthoelase and microcline. The albite also appears to approach 

 more closely to oligoclase than was supposed. 



This is the typical composition of a granite fairly rich in soda and 

 lime. Granitic rocks with a closely-similar composition have been 

 described as granitite. It is included in the subrang lip arose 

 of the American classification. 



Only some 20 yards or so on the up-stream side of the islands 

 where the rock last described was exposed, is a small grey rock 

 (M15, PL Y, figs. 5 & 6 ; specific gravity = 2-63 to 2-69), visible 

 only when the river is low. In hand-specimens this presents a 

 saccharoidal appearance like a crystalline limestone. In colour it 

 shows various shades of dull green arranged in lamina?, so that 

 it has a distinctly-streaky appearance. Under the microscope, it 

 is seen to have in most places the characteristic structure of a 

 granulite — consisting of an aggregate of small, more or less rounded 

 grains of quartz and microcline with a considerable amount of 

 felspar, which only occasionally shows twin-lamellation, but is 

 probably in most cases albite. Some of the grains of quartz appear 

 to be idiomorphic. The average diameter of the minerals is about 

 50 microns (0*05 millimetre). A few larger crystals of albite are 

 met with, containing numerous needle-like inclusions, which have a 

 birefringence of more than 11 thousandths. The faster vibrations 

 of these inclusions are at right-angles to the length, and they are 

 perhaps a hydrous mica. 



There are numerous crystals of a pale-green granular augite, 

 usually comparable in size and shape with the quartz and felspar 

 described above, although a few may measure half a millimetre in 

 diameter. They show little pleochroism. Some of the individuals, 

 especially those sections which are near the clinopinakoid, are not 

 quite dark in the position of extinction. This is presumably due 

 to the dispersion of the bisectrices for the different colours. The 

 birefringence reaches 20 thousandths. 



There are also occasional crystals of sphene, which are strongly 

 pleochroic, changing from a pale grey-green (something like that 

 of the augite) to a dull orange. The birefringence exceeds 85 

 thousandths. 



Hornblende occurs in small lath-shaped crystals or elongated 

 plates, with an extinction-angle not exceeding 19°. It is strongly 

 pleochroic, changing between a pale green, identical with that of 

 the augite, and a deep blue-green. 



In some places there are extensive aggregates of felspar, of much 



