C. HE. Tilley—Para-Gneisses in South Australia. 307 
maximum value which is a function of temperature, and this 
maximum stress diminishes with rising temperature, so that at very 
high temperatures the stress element in metamorphism may be quite 
subordinate. During the metamorphism of the sediments under 
consideration, and at the temperature prevailing, shearing stress 
was adequate to orient biotite, quartz, and flakes of graphite, and 
in some cases elongate individual garnet crystals during their 
crystallization in the rigid garnet gneisses, but only a granular 
texture was imparted to the dolomite rocks. 
Doubtless in general an important factor in this different 
behaviour is the position of dolomite with regard to quartz and 
biotite in the crystalloblastic series. 
That at a given temperature the upper limit of shearing stress 
is lower for the carbonate rock than for the more rigid quartza- 
felspathic type, seems undoubted, and the essential factor in this 
behaviour is the ease with which the carbonate minerals 
recrystallize, and the consequent tendency to a viscous flow limiting 
the range of shearing stress. The presence of carbon dioxide as a 
solvent in rocks of this class contributes to this end. 
As to the nature of the original sediments from which these paraee 
gneisses are derived, all traces of original clastic structure have been 
destroyed by the complete recrystallization involved. Nor in the 
nature of the case are the evidences of progressive metamorphism 
by which the succession of mineralogical changes might be 
determined clearly defined. The mineralogical composition of 
the group is that which might be expected in the metamorphism 
of sediments of the composition of shales, grading into more 
quartzose types, and even merging into arkoses, where the 
felspathic constituent is dominant. 
The two prime constituents of all the rocks are quartz and 
felspar, and the important additional minerals are biotite, garnet, 
sillimanite, and spinel. 
Sleaford Vermont 
Analysis. gneiss. slate. Norm. Osann class. 
SiO, 64:35 62°37 Quartz S = 70-4 
AlgOg 16°21 15°43 21°7 AV bro 
_ Fe203 101 1°34 Orthoclase Ce 6 
FeO 6-91 5°34 311 F = 12:0 
MgO 2°95 3°14 Albite M= — 
CaO 1°35 0°77 14-1 T= 34 
Na,O 1:70 1:14 Anorthite 1 = 11953 
K,0 5°25 4:20 6°7 
H,O+ 0:32 | 3-71 Corundum By = byes 
H,O — 0-14 0°34 5:4 @ = Ile7 
other Hypersthene f = 12°5 
constituents n.d. 2°02 19°3 
Magnetite 
100°19 99°80 1-4 
