NOTES ON ANTARCTIC ROCKS. 473 
that we have found, among some sections cut for stauroscopic 
investigation, a specimen of topaz in which the optic axes make a 
very small angle with each other (not more than a few degrees) 
and that the optical sign is negative. It may be that under 
certain conditions topaz changes its axial angle, and after passing 
through zero the axes open out in a plane at right angles to their 
former position with changed sign. 
Besides some apatites and zircons there are some grains with 
extremely strong double refraction. One of these, a small prism 
showing pyramid faces at one end, gives colours of the seventh 
order in polarized light (Plate 13, fig. 3). The presence of some 
prismatic cleavage lines render it probable that it is rutile. 
There are a few tabular crystals of anatase, and some grains, 
with somewhat lower double refraction and without cleavage, may 
most probably be referred to cassiterite. : 
Trachyte, (Nos. 1 and 2). 
Compact, greenish-grey in colour, somewhat fissile. Sp. gr. 2°49. 
Analysis yielded the following composition :— 
SiO, = 61-01 
Al,O; = 16°62 
Fe,O,; = 3°59 
FeO = 281 
MnO =. "00 
CaO a Bal : 
MgO es” 08 
Na,O = $92 
K,0 & 5°22 
Water (ignition)= 1°13 
100714 
Phosphoric acid and chlorine present in small quantities. 
Under the microscope the rock is seen to be composed principally 
of sanidine microlites. There is a small proportion of lath-shaped 
triclinic microlites and of cryptocrystalline interstitial material. 
