306 C. £. Tilley —Para-Gneisses in South Australia. 
That the rocks have developed under a not inconsiderable shearing 
stressis evidenced by the development of biotite, quartz and graphite _ 
in flakes elongated in a plane of foliation, which is here coincident 
with the banding surface, thus serving to accentuate the banding. 
We have already referred to the development of garnet in lenticular 
grains with noteworthy elongation parallel to the foliation, and to 
the occurrence of uniformly oriented sillimanite. 
This streaky development of sillimanite in orthoclase is strongly 
suggestive of development under actual shearing movement. 
Granulation of the enclosing mineral might be expected in such 
cases, but it is quite clear that if this movement has taken place 
the evidence of granulation of the enclosing felspar has been 
destroyed by healing. The most probable explanation appears to be 
that crystallization of sillimanite occurred under conditions 
permitting shearing movement, and at temperatures sufficiently high 
to inhibit granulation of felspar. 
To the case of garnet the same remarks may apply. The lens-like 
character of the garnet porphyroblasts, with their longer axes 
parallel to the’ foliation, is a feature of a number of the Sleaford 
gneisses. Here, again, the enclosing mineral shows perfect optical 
continuity at the periphery of the garnet grains. However this may 
be, it is quite clear that in many of the garnet grains the elongate 
character cannot be due to any rolling out of original garnet crystals, 
and in so far as this is true, it shows that the rocks have 
been developed under sufficient shearing stress to elongate a mineral 
so high in the crystalloblastic series. Even in the case of certain 
abnormally elongate garnet lenticles, it must be conceded that any 
postulated rolling out was involved in the same epoch of meta- 
morphism, and before the temperature had fallen sufficiently to 
allow of granulation. Shearing stress has therefore played a not 
unimportant role in the development of these gneisses. The element 
of stress has also left its mark after crystallization had been 
completed, the clues to which are the development of rows of fluid 
pores in quartz grains at right angles to the direction of foliation, 
and in the case of the Hutchison garnet gneisses, the extensive 
cataclasis. This cataclasis is associated only with the gneisses of the 
northern area, and is absent in the Sleaford rocks. 
It was pointed out in a previous paper that the dolomites 
associated with these gneisses showed no evidence that shearing 
stress had played any important role. The propriety of these two 
apparently conflicting statements calls for some remark. In the 
case of pure dolomites we are concerned with a mineral whose habit 
is not favourable to the development of elongate crystals, and there 
is a further factor involving the facility with which the carbonate 
minerals recrystallize. With regard to mineralogical evidence of 
shearing stress, the abundant presence of the anti-stress mineral 
forsterite indicates that the hydrostatic element was dominant. 
As has been pointed out by Harker,! shearing stress has a 
1 Harker, Pres. Address Geol. Soc., 1918, p. Ixxix. 
