256 THE COXSTITUTIOX OF CERTAIN ROCKS OE THE ARCH.-EAN AGE. 



appearance to that in the gneiss, but is of course not drawn out: 

 into lenticles. The crystals have crystallised in allotriomorphic 

 forms. There are numerous liquid inclusions present in the quartz^ 

 but on the whole comparatively few of the particles show the 

 effects of straining- to anything- like the extent that those in the 

 g^neiss do. Numbers of quartz crystals show no effects of strain- 

 ings at all. The felspar group is represented chiefly by microline. 

 The cross-hatching- is distinctly visible. Decomposition is not sO' 

 marked as in the case of the g-neiss. Still there are but few of the 

 felspar crystals that do not show the effects of straining by their 

 optical behaviour. Albite is also present. The mica is the variety 

 biotite and the hornblende is a dark greenish black variety. The 

 decomposition products in the felspars appear to be Kaolinite. 

 Occasionally may be seen instances of the interlaniination of the 

 members of the felspar group, thus giving rise to the micro- 

 perthite structure. In addition to this, examples may be found of 

 the intergrowth of felspar and quartz, producing a micropegmatite 

 structure. On the whole the general agreement of the mineralo-- 

 gical constituents of the underhing or associated granites with 

 those of the gneiss lends considerable support to the theory 

 hitherto mentioned by me that the gneiss, which forms a sort of 

 backbone to the rocks in this part of the country, was derived' 

 from the granites themselves through the effects of mcfamorphism ' 

 produced or to a great extent produced by terrestrial disturbances. 

 Reference was made by me, in my last discourse, to the Tact that 

 these granitic rocks had been intruded by basic rocks at two • 

 different periods. Although full reference was then made to the- 

 macroscopic features, little could be said as to the real mineral' 

 constituents or nature of the intrusive rocks owing to their state - 

 of decav. However, I have now unearthed portions of these rocks 

 to permit of their determination. The older type of igneous intru- 

 sive rock consists of an admixture of hornblende, felspar and ' 

 quartz. The hornblende is bv far the most plentiful constituent. 

 In fact so great is its presence that, to the naked eve, Ihe rock 

 presents the appearance of a dark basic crystalline rock. The 

 hornblende is a dark greenish black variety. In fact it is the 

 same variety that pervades the granite and gneiss. Except for 

 the occurrence of a flake of biotite here and there, mica may be 

 regarded as absent. Twin crystals of hornblende are sometimes- 

 to be seen. The hornblende also shows a high relief, strong 

 plcochroism and the cleavage angle reaches 124°. Whilst irr 

 the granite, and more conspicuously in the gneiss, the felspar 

 shows twinning on the albite tvpe and at times most markedly 

 both on the albite and pericline types, in this type of rock pericHne 

 twinning in the felspars is conspicuous by its absence. In fact, 

 many of the felspars here present show but a meagre trace of 

 twinning on the albite type. Microclinc is not present, so far as 

 my examination shows. The rock is holocrystalline. Next In 

 abundance to hornblende may be mentioned the felspars, which 

 are whollv triclinic in variety. The hornblende, following the usual 

 rule of Rosenbusch as to basicity, has preceded the felspar irr 

 the order of crystallisation. The felspar, quartz and some of the 

 hornblende has crystallised in an allotriomorphic form, but some- 



