116 



Apatite is a fairly common accessory. Macroscopically 

 it is greenish-white in colour, forming granular aggregates 

 of a pale-greenish colour and iip to an inch in diameter in 

 the pegmatite. It is microscropically a pale-grey-brown in 

 colour with faint absorjDtion E > 0. Its determination was 

 confirmed by grinding a little with nitric acid and ammo- 

 nium molybdate, when the characteristic yellow phosphomol- 

 ybdate was formed in abundance. In the rock of the main in- 

 trusion it occurs in irregular or oval grains with a fairly well- 

 marked basal parting. It is often faintly pleochroic in tints 

 from grey- violet to grey-brown. It would in general appear 

 to have crystallized before the ilmenite. Secondary epidote 

 sometimes surrounds the grains. 



Sphtae occurs in a few of the slides cut from Houghton 

 rocks. The pegmatite for which the name "yatalite" has been 

 suggested contains a considerable amount of this mineral. 

 Macroscopically it is present in pale-green to brownish-green 

 crystals, with irregular outlines, but with two good cleavages. 

 Microscopically it is light-brown by transmitted light. In 

 convergent light the optical axial angle is seen to be in gene- 

 ral unusually small. Some crystals, however, occur whose 

 axial angles are of the normal size, and show the great dis- 

 persion well. The mineral is positive optically and weakly 

 pleochroic. In yatalite is occurs in grains up to ^ in. in 

 diameter, or smaller grains included poikilitically in the ac- 

 tinolite. In the rock of which an analysis is given below it 

 occurs in rounded to subidiomorphic grains with the usual 

 prism cleavages and traces of another cleavage. In one or 

 two instances polysynthetic twinning is developed. 



Quartz occurs irregularly, generally interstitially. It 

 nearly always shows strain efforts, and contains liquid inclu- 

 sions. Very thick sections appear sometimes to be twinned. 



Epidote is generally present in most instances. It is usu- 

 ally recognizable in hand specimens by its characteristic yel- 

 low-green colour. Microscopically it occurs in large irregular 

 grains in the uralite and dusting over the felspar. It is 

 formed during the decomposition of the former and by its 

 interaction with the latter. Cleavage is rarely well developed. 



Sea polite occurs in a single specimen, and its peculiar 

 nature is described later (p. 124). 



Siderite is an accessory in one specimen, and is also de- 

 scribed later on (p. 118). 



a. Texture. 



Considerable variety of texture is presented both macro- 

 scopically and microscopically by the intrusive rocks of this 



