1]2 
Rex T. Prider. 
alkalies, especially potash, are also slighter lower. These features indicate 
the close resemblance of the two rocks and also indicate that there has been 
no addition of material to the xenoliths from the granite gneiss. 
(ii) Hornhlendic schlieren . — These small lenses in the upper gneiss were 
■only noted in one place (40 chains N., 527 chains W. from datum). They are 
irregular shaped len tides up to 12 inches wide, which are elongated parallel 
to the foliation of the enclosing gneiss. 
That these schlieren are older than the intrusive granite gneiss is shown 
by the narrow granitic veinlets penetrating them, and also by the fact that 
small fragments of the xenolithic material are found in the enclosing gneiss. 
The central }3ortion of these inclusions is made up largely of a dark green, 
bladed hornblende, in crystals up to 0-5 cm. long, with occasional larger 
plates of a pale greenish pyroxene. Small white angular patches and veinlets 
of felsjDathic material are present. Near the edge of the xenoliths the rock 
becomes lighter in colour, hornblende becomes rarer, a pale greenish pyroxene 
taking its ]>lace. The enclosing gneiss is a fine granulitic microcline granite 
gneiss. 
Under the microscope the fine microcline gneiss is seen to have pale 
gi*eenish liornblende derived from the hornblendic xenolith, as its ferromag- 
nesian. Approaching the contact the most noticeable features are (1) the 
increase in the hornblende content, (2) entrance of idioblastic sphene and 
colourless pyroxene, (3) decrease of microcline and corresponding increase 
of oligoclase. The felspars at the actual contact are considerably epidotised. 
The xenolith itself is made up mainly of pyroxene, hornblende, and 
introdiiced quartz-oligoclase veinlets. The outer zone of the xenolith is 
rich in pyroxene (in large plates up to 4 or 5 mm. diameter), which under the 
microscope are seen to be fringed with pale greenish hornblende. The pyroxene 
in thin section is colourless, with 110 cleavages well developed and a marked 
parting ]>arallel to 100 and less j^erfect parting on 010. 
The properties Z a c = 42° ; y = 1-705 ; « = 1*676 ; y - a = 
•029 ; biaxial (+), and very weak dispersion, point to a diopsidic pyroxene 
with 20 per cent, of the hedenbergite molecule. 
The amphibole which is clearly developing from the ])yroxene is a pale 
greenish, somewhat fibrous va^-iety, with an irregular coloration in the same 
crystal. Pleochroism is X yellow-green ; Y olive green ; Z bluish-green ; 
and absorption X<Z<Y. Z A c ^ 23°. This is a common blue-green horn- 
blende. It is usually developed where the pyroxene is in contact with oligo- 
clase and rarely where in contact with quartz, the hornblende having apjmrently 
obtained its alumina from the felspar. 
In the centre of the xenolith, although most of the diopside has been 
replaced by amphibole, cores of diopside in this mineral [)oint clearly to its 
origin. 
These xenoliths have been originally dio]}sidic rocks, now converted into 
rocks composed ]3redominantly of hornblende. An anomalous feature is that 
the cores show considerably more alteration than the periphery. There has 
been a slight hybridisation of the adjoining granite (gneiss) giving rise to a 
fine-grained hornblende granite gneiss. This hybridisation of the granite is 
developed on a larger scale in the Lower Gneiss and this ciuestion will I)e 
considered more fully at a later stage. 
(iii) Biotite granidUe xenoliths . — These are much finer-grained and carry 
more ferromagnesians than the enclosing gneiss, in which they occur as 
irregular shaped elongated darker-coloured patches, in which the foliation 
approximates to that in the surrounding gneiss. All these rocks are alike 
