PETROLOOICAL NOTES ON FURTHER ROCK SPECIMENS. 323 



Two of the rocks, viz., Nos. 783, 792, are like No. 794 of Stillwell in that they have 

 two plagioclases present. In No. 783 there is a very small quantity of plagioclase which 

 has Da = 1-538 (approx.). Accordingly its chemical composition is oligoclase \\ith 

 Ab 80 An 20 . Most of the plagioclase in this rock gives a maximum symmetrical 

 extinction (X' A 010) of 20, showing it to be andesine with Ab^ An 37 . In No. 792 

 some of the plagioclase has n a 1-535 and n Y 1-545. This determines its composition 

 as oligoclase with Abgg An 16 ; but here again, most of the plagioclase is more basic. 

 The extinction (X' A 010) is 28, corresponding to labradorite, Ab 48 An 52 . 



The other two rocks, viz., Nos. 775, 788, carry only the more basic plagioclase, 

 labradorite, whose composition is approximately Ab so An 50 . 



The felspar in each of the four rocks shows both albite and pericline multiple 

 twin lamellae. In every case the twinning is indistinct and tends to be discontinuous 

 along the length of the laths. No. 792 has the least indefinite twin forms found in 

 this suite of rocks, but even so there is a tendency towards the elimination of twinning 

 in the central portions of the plagioclase grains. In No. 775 the concentric circular 

 nature of the pericline twin lines is an indication of the deformations produced in this 

 mineral by stresses. 



Other stress effects are evidenced by the undulose extinction and the mortar 

 structure associated with the peripheral granulation of the felspar. 



None of the rocks shows saussuritised felspar, but every one contains felspar which 

 holds inclusions of several kinds. No. 775 has abundant inclusions of magnetite dust, 

 indiscriminately arranged flakes of biotite and green shreds of hornblende, and some 

 crystals of zircon. No. 783 is much the same. The felspar of No. 788 is much cleaner, 

 being practically free from hornblende and biotite inclusions, although it still has some 

 magnetite and shows a new complication, viz., the presence of granulated pyroxene 

 inclusions. There is a recurrence of the biotite flakes in No. 792, but here they tend 

 to be arranged as tongue-like forms along cracks and cleavage lines rather than 

 indiscriminate distributions through the grains as in No. 775. Magnetite dust is present 

 in this specimen and also small granules of pyroxene (cf. No. 788), which are concentrated 

 in the vicinity of larger grains. 



These four rocks, like No. 794, contain both monoclinic and orthorhombic 

 pyroxenes, the latter always being in considerable excess. 



The orthorhombic pyroxene in No. 775 has np = 1-710 (approx.), D.R. = 0-014. 

 Its elongation is positive, its optic sign negative, 2V is large. These characteristics* 

 are those of the hypersthene member of the enstatite-hypersthene series which carries 

 a molecular percentage of 30 of FeSi0 3 . In No. 783 n a is slightly less than 1-680, which 

 shows that the ratio MgSi0 3 : FeSi0 3 is greater, there only being 20 per cent. FeSi0 3 

 present. The mineral is still hypersthene. 



* Wfaohell, 1927. VoL II, p. 177. 

 80-C 



