312 CO. #. Tilley—Para-Gneisses in South Australia. 
containing this mineral, and in the Phillipsburg Quadrangle, 
Montana, Calkins and Emmons! have described garnet-cordierite 
gneisses of which the same mineral is a constituent. The modes of 
occurrence, and associations of all these rocks, indicate that spinel 
is a product of the highest grades of metamorphism, and appears in 
rocks subject to intense thermal alteration or as members of 
Grubenmann’s Kata -zone. 
SUMMARY. 
1. Garnet gneisses of sedimentary origin are described from 
the oldest group of sediments recognized in Southern Hyre 
Peninsula, South Australia. The high-grade metamorphism 
which these rocks have undergone has resulted in the development 
of felspar, garnet (almandine), sillimanite, and green spinel. 
2. The mineralogical association of these constituents is 
described, and their genetic relations are discussed. 
3. A sedimentary origin of these gneisses is attested by their 
interbanding with undoubted sediments, their mineralogical com- 
position, and lastly by chemical analysis. 
4. In the garnet gneisses developed in the Grenville series of 
North America, notably those of the Adirondack region, and in the 
province of Quebec, these para-gneisses find their most ’striking 
analogues. 
EXPLANATION OF PLATE III.* 
Fie. 1.—Garnet-sillimanite Gneiss, Mine Creek, Hundred of Hutchison. 
x 32 diameters. The constituents are garnet, biotite, silliimanite, ortho- 
clase, and quartz. The features illustrated in the photomicrograph are 
the development of elongate garnet porphyroblasts, the poikiloblastic 
texture, and the transverse fissures in the garnet occupied by biotite, the 
degradation product. Sillimanite is also seen in bunched aggregates 
included in orthoclase. 
Fic. 2.—Garnet Gneiss, Mine Creek, Hundred of Hutchison. x 32 diameters. 
The constituents are garnet, biotite, sillimanite, orthoclase, plagioclase, 
quartz, green spinel, and magnetite. The features noted in the photo- | 
micrograph are the development of a system of cracks in the garnet 
grains, parallel to the foliation of the rock, and the association of green 
spinel and sillimanite (spinel included within sillimanite in one case) as 
inclusions in the largest garnet crystal shown. 
1 Emmons and Calkins, Prof. Paper No. 78, U.S.G.S., 1913, p. 39. 
2 Issued with June number. 
