16'2 THE PYROXENE GRANULITES COLLECTED 



analyses it has an exceedingly high percentage, viz., 1L37. This high percentage of 

 lime is justified by the fact that all the minerals of the rock (quartz excepted) contain 

 a considerable amount of CaO. The alkalies are fairly normal in analysis No. I, but 

 are almost absent in No. II, their place here being taken by the CaO. The analyses 

 strongly suggest that these rocks belong to Rosenbusch's group of Para-gneisses. 



The pyroxene granulites are typically of a greenish colour — some light green and 

 others much darker. They are fine-grained to medium-grained, and the grains are 

 very even in size. They are very hard, much silicified, and have a somewhat greasy 

 lustre. The degree of foliation varies considerably ; in some of them there is no folia- 

 tion present, while in others it is very well marked. The fracture is always smooth 

 and approaches conchoidal. Some of the constituent minerals can generally be identified 

 in hand-specimen — notably quartz and sometimes felspar. The ferro-magnesian 

 mineral, too, is generally recognisable as such, though its more exact determination is 

 difficult. The light- and dark-coloured minerals are distributed fairly evenly throughout 

 the rock, and in a few cases there are coarse veins of quartz running through the hand- 

 specimens. 



These rocks may conveniently be divided into three classes, as follows : 



(1) Acid pyroxene granulites. 



(2) Scapolite-bearing pyroxene granulites. 



(3) Basic pyroxene granulites. 



The acid pyroxene granulites are those in which quartz is present in large quantity. 

 The presence, in fair amount, of scapolite in a number of the rocks has led to their 

 being classed together as a separate group. The basic pyroxene granulites are those 

 in which quartz is absent or only present in subordinate amount. It is somewhat 

 difficult to place definitely the dividing-line between the acid and basic groups. The 

 amount of quartz present decreases fairly regularly from the most acid down to those 

 where it is quite absent. 



The mineralogical composition of these rocks is shown in the table on p. 163. 



(1) THE ACID PYROXENE GRANULITES 



This class includes those granulites in which quartz is present in abundance. The 

 constituent minerals are quartz, pyroxene (monoclinic), felspar, sphene, zircon, apatite, 

 magnetite, sericite, and carbonate. The quartz, in almost all cases, shows shadowy 

 extinctions and has suffered a certain amount of granulation. The pyroxene is in 

 all cases monoclinic. It is colourless and only exhibits a slight amount of absorption. 

 It is characterised by a fairly uniform high extinction (35° to 40°) and 2V is large. It 

 is, then, almost certainly, diopside. The pyroxene is the. same throughout both the 

 acid and basic groups and also in most of the scapolite-bearing group. The felspar is 

 plagioclase, and where suitable sections for its specific determination are present it is 

 found to be labradorite. Sphene and zircon are constant members, and occur in 

 characteristic forms. Apatite, magnetite, and biotite are occasional constituents. 

 Sericite and carbonate, and possibly also some of the biotite, are secondary. 



96. Pyroxene Granulite, Erratic (Cape Royds). 



In hand-specimen this is a dark-coloured rock, very hard and much silicified. It has 

 a smooth, conchoidal fracture. It is extremely fine-grained and is noticeably banded. 

 Quartz and a light-greenish ferro-magnesian mineral can be recognised and also occasional 

 small flakes of biotite. 



Microscopically the rock is seen to have a granulitic fabric ; there is a tendency to 



