CO, EB. Tilley—Para-Gneisses in South Australia, 311 
In the garnet-cordierite-sillimanite gneisses the iron-magnesia 
constituents of the rock are allotted between garnet, cordierite, 
and biotite. 
The equilibria between these minerals can be represented by the 
scheme 
(c) Biotite, orthoclase 
sillimanite, water 
v1 
(b) (Biotite, muscovite, quartz) 
Wee IN 
ZN 
(d) Almandine, orthoclase (a) Cordierite, orthoclase 
sillimanite, water sillimanite, water 
and the minerals of the systems (a), (c), and (d) are found together. 
In the rocks under discussion, however, there has been some pre- 
vailing condition by which the equilibria have been driven com- 
pletely from (a) to (c) and (d). 
Whether this is connected with the very high ferrous iron content 
of the rock compared with magnesia, and the possible instability 
of an iron-rich cordierite, must await further investigation. 
Correlation of the garnet gneisses with similar gneisses in other areas. 
Rocks of this type can be matched in pre-Cambrian tracts in 
other parts of the world. The most striking analogues are the garnet 
gneisses of the Grenville series of North America. 
Adams ! describes garnet gneisses of the sediment type associated 
with crystalline limestones in the region north of the island of 
Montreal, Quebec, and more recently in the Haliburton-Bancroft 
area of Ontario.” 
These rocks are extensively developed in the Grenville series in 
the Adirondack region of New York, and have been described by 
Kemp,? Smyth, Cushing, and others. 
The field relations and petrographical constitution of the rocks 
described by these investigators are remarkably similar to those 
of the rocks described in the foregoing. 
Metamorphosed sediments of this type containing a green spinel 
have been studied from various localities. Spinel is developed in 
the well-known contact rocks surrounding the tonalite of the Mount 
Aviolo region in the Eastern Alps as described by Salomon.* The 
references to the earlier literature will be found in a paper by J. J. H. 
Teall. More recently Sauerbrei ® has studied sedimentary gneisses 
1 Adams, Amer. Journ. Sci., vol. 1, 1895, pp. 58-69. 
* Adams and Barlow, Mem. No. 6, Geol. Surv., Canada, 1910, pp. 173-91. 
> Kemp, Amer. Assoc. Adv. Sci., 1900, pp. 157-84. 
4 Salomon, Zeit. d. deutsch. geol. Gesell., vol. xlii, 1890, s. 450. 
° Teall, Proc. Geol. Assoc., vol. xvi, 1899, pp. 61-74. 
§ Sauerbrei, Newes Jahrbuch, vol. xxxiv, 1912, pp. 1-41. 
