PETROLOOICAL NOTES ON FURTHER ROCK SPECIMENS. 



325 



This biotite is associated with pyroxene, felspar, and ilmenite, frequently, but 

 not always, with hornblende, and in No. 775 at least, with scaly calcite : it is 

 intimately associated genetically with these associates, but see below, part (c). 



It is always secondary (also see part (c)). In places, particularly in Nos. 783, 792, 

 there are crystals whose curved outlines with sweeping bends suggest change of direction 

 during growth produced by the variation of the prevailing stress-direction. 



The magnetite (ilmenite) present in all these rocks has often been referred to 

 above. Its mode of formation and other relationships to the remaining minerals will 

 be treated below, see part (c). 



(c) Critical Analysis of Genetics of the Gneiss. 



We can assume that the original rock was a felspar-pyroxene one. If we suppose 

 that the metamorphic products, M, represent both original felspar and augite, and that 

 all the iron ores came from the augite (some, of course, may have been original), and 

 the amphibole and mica came equally (the simplest assumption, though, necessarily, 

 unsubstantiated) from the original minerals, the modal compositions of the parent rocks 

 are as follows : 



We can justifiably assume that the original plagioclase was basic, a labradorite. 

 It is not so easy to tell if the orthorhombic pyroxene is original, although Hatch (op. cit., 

 p. 415) suggests that hypersthene-gabbro, hypersthene-diorite and hypersthene-granite 

 may possibly be formed by differentiation, which implies the original character of the 

 hvpersthnio. If it is true as has been suggested (Tyrrell, 1930, p. 139) that one of the 

 distinguishing features of the charnockite series rocks from norite to pyroxene-granite 

 is the poverty of water-formed or water-rich minerals such as biotite and hornblende, 

 then an accession of water must be postulated to account for the presence of these minerals 

 in the present rocks. 



3. THE HYPERSTHENE-ALKALI-FELSPAR-GNEISS. 



(a) Macroscopic Features. 



Stillwell (p. 133, et seq.) has dealt with Specimens Nos. 795, 797 of this series. 

 There remain Specimens Nos. 778, 779, 787, 790, 791, 793, 798, 1226, 1227, 1254 to be 

 described. 



