18 



No. 1, and there is a little pyrites. Apatite is in tiny needles 

 and in stout prisms about '25 mm. long. A few of the crystals 

 are partially coloured, and pleochroic in blue and brown tints. 



Occasional large crystals of microcline, opaline quartz, 

 and plagioclase are scattered through the rock. 



No. 4 is slightly coarser than the preceding type, but 

 similar to it in fabric and mineral constitution, except for the 

 absence of hornblende. The variation in grainsize is between 

 wider limits, particularly in the plagioclase. Irregular patches 

 of an opaque mineral with a dirty buff colour and chalky 

 appearance in reflected light, often associated with ilmenite, 

 may be leucoxene. Small grains of microcline are sparingly 

 present, but more numerous than in No. 3. 



In Nos. 5 and 6 a progressive increase in the grainsize of 

 the body of the rock is shown, as well as in the proportion of 

 larger crystals ; these latt-er also increase in size, the microcline 

 in No. 5 attaining a length of 4 cm. Apart from these larger 

 crystals the rock is medium to fine grained. Biotite now 

 occurs in relatively large flakes about 6 mm. in diameter. 

 The ground-mass consists mainly of plagioclase, quartz, and 

 biotite, with subordinate microcline. Apatite, ilmenit-e, and 

 zircon are minor accessories, and appear to be confined princi- 

 pally to the larger flakes of biotite. 



The rock might well be termed an AdameUite porphyry 

 (pi. iii., fig. 1). 



In slides 3 and 4 there is evidence of corrosion in the 

 opalescent quartz ; the contours are rounded and, as it were, 

 mitred and surrounded with a ring of tiny quartz granules 

 forming part of the ground-mass, but optically continuous with 

 the large grain (pi. i., fig. 3). The plagioclase crystals, too, 

 have been cracked and faulted, and the edges "nibbled" by the 

 ground-mass. PI. iii., fig. 1, represents well the rounded con- 

 tours of a large microcline crystal in slide No. 6, evidently due 

 to corrosion. Microscopic examination shows that this very 

 crystal along part of its margin has been altered into plagio- 

 clase of about the same R.I. as quartz, and so probably 

 oligoclase or andesine. 



Apart from these superimposed characters the large 

 crystals are similar in every respect to those of the porphyritic 

 granite. 



These circumstances, together with the fact noted 

 above, that in the inclusions of diorite in the granite the 

 proportion of large crystals increases towards the margin, 

 indicate that these large crystals have been derived in some 

 way from the magma of the porphyritic granite. 



One is reminded, on examining the diorite and these 

 transitional rocks, of the basic segregations described by 



