33 



Assuming the deficiency from 100 per cent, in each case 



to be represented by kaolin, the mineral constitution is 

 approximately : — 



A B 



Quartz 41-9 54*5 



Albite 42-4 14-6 



Muscovite 8'0 27-1 



Kaolin 7*7 3-8 



Now if, assuming the albite to be pure, one attempts to 

 calculate the percentage composition of the original grano- 

 phyre from the partial analyses given, by substituting albite 

 for corresponding volumes of muscovite and quartz, and 

 changing the kaolin to albite, the figures found in each case 

 are very close, the mean of the two calculations being: — 



SiO. 76-1 



AlA 14-9 



Na^^O ; ... 9-0 



All these calculations are, of course, only very approx- 

 imate, but they do point to the conclusion that the griesenised 

 rock was derived from a soda granophyre essentially similar in 

 chemical composition to the soda aplite. 



(9) URALITIC DOLERITE. 



With one exception, to be presently referred to, the 

 rocks from all the dykes examined bear a close resemblance to 

 each other in general mineralogical characteristics. They are 

 all fine grained, and the megascopic appearance varies in differ- 

 ent specimens from that of a dense basalt to that of a fine- 

 grained dolerite. 



The principal constituents are felspar and uralite. 

 Plagioclase is in fairly perfect laths and quite fresh ; it is 

 a labradorite about Ab^^. Augite has been entirely converted 

 into amphibole, rather a pale-green colour for the most part, 

 sometimes in plates dusted with abundant secondary mag- 

 netite, but very often in aggregates of needle-shaped crystals ; 

 a little tremolite is also seen. Dense mats of the amphibole 

 occupy the spaces between the felspars and often project into 

 the latter. Iron ore — ilmenite by its habit — is plentiful, and 

 a little biotite is always present, as well as apatite in exceed- 

 ingly fine needles. There is no indication that olivine ever 

 formed part of the rock. 



Occasionally, as in the dolerite from the meridional dyke 

 through the schists at the back of Rosetta Head, relict ophitic 

 fabric is well shown, the amphibole in large plates enclosing 

 the felspar. 



b2 



