Granite-Gneisses of Southern Eyre Peninsula. 199 



liere ; resolution of these has produced the more important struc- 

 tures observed. This idea of torsional stresses has been developed 

 for a larger area in North Wales by one of the authors, in con- 

 junction with Mr. C. B. Wedd. The facts given here may serve 

 as detailed evidence when Mr. B. Smith & Mr. Wedd publish their 

 theories. 



The concertina-folding in the sjmclinoria appears to be related 

 to the tough anticlinal nodes of the northern Ordovician outcrops. 



The master-faults separate blocks of country which appear to have 

 been displaced laterally. These faults originated in the Devonian 

 Period, but have moved, conspicuously in post- Carboniferous times. 



The minor faults appear to be adjustments that allow the strata 

 to comply with torsional stresses. Here again lateral movement is of 

 importance. The twisting-round of some of the fault-blocks is de- 

 monstrated by the abnormal strike of the beds and of the cleavage. 



February 2nd, 1921.— Mr. E. D. Oldham, F.E.S., President, 

 in the Chair. 



The following communications were read : — 



1. A New Species of Blattoid (Archimylacris) from the Keele 

 Group (Stephanian) of Shropshire. By Herbert Bolton, M.Sc, 

 F.E.S.E., F.G.S. 



2. 'The Granite-Gneisses of Southern Eyre Peninsula (South 

 Australia) and their Associated Amphibolites.' By Cecil Edgar 

 Tilley, B.Sc, A.I.C. 



The great tract of Southern Eyre Peninsula is underlain by a 

 complex series of pre-Cambrian rocks, subject to prolonged erosion, 

 but now in large part covered by a mantle of weathered products 

 and recent seolian sediments. An introductory summary is given 

 of the several divisions of the pre-Cambrian terrain of the penin- 

 sular area. The fundamental platform of the eastern half of the 

 peninsula consists of granite-gneisses, amphibolites, and hornblende- 

 schists, embraced within the Flinders Series. 



The acid members of that series consist of biotite, biotite-horn- 

 blende, and hornblendic granite-gneisses, pyroxene-bearing granites 

 including charnockitic types, and a great development of garnet- 

 gneisses. The satellitic pegmatite phases are predominantly 

 hornblende-pegmatites, in which diopside may be a not infrequent 

 constituent. 



The petrography of these rocks is described, and the significance 

 of their mineralogical constitution (in the presence of garnet, 

 diopside, etc.) discussed. The gneissic structure, which is a 

 characteristic feature throughout the area, is a, primary gneissic 

 banding arising from flow-movements in a heterogeneous magma. 



The basic members of the Flinders Series comprise amphibolites, 

 pyroxene-amphibolitcs, garnet-amphibolites, pyroxene-granulites, 

 and quartz-orthoclase-amphibolites. These amphibolites represent 

 earlier consolidated rocks incorporated and engulfed within the suc- 

 ceeding acid magma. They have the mineralogical composition oi 

 basic igneous rocks, and have suffered thermal metamorphism bv 

 the invading granite-gneisses. The quartz-orthoclase-amphibolites 

 have the chemical composition of quartz-gabbros, In a large 

 amphibolite-band of this type, the central portion is a massive, 

 partly-uralitizcd, micropegmatitic diabase, while at the periphery 



