Reports & Proceedings — Geological Society of London. 381 



interbanded with gneisses ; and, as the latter are mainly of igneous 

 origin, they are thought to be intrusive into, and therefore younger 

 thau, the schists. As a general rule, the foliation and the banding of 

 the gneisses are well defined in parallel uncontorted planes, the 

 strike being commonly along, or somewhat north of, a north-east 

 to south-west direction. In certain inselberg peaks, the strike sweeps 

 round the contours, while the foliation surfaces dip quaquaversally 

 from the summit. Into the gneisses later granites, belonging to at 

 least two different periods, have penetrated, riddling them with 

 enormous numbers of small intrusions, lit-par-lit injections, tongues, 

 and apophyses, llocks of later age are rarely met with ; but in 

 a few places dykes of picrite and pyroxenite have been found cutting 

 the youngest pegmatites. 



The succession of rocks in eight of the better-known districts is 

 described, and the following general classification is based on the 

 details thus provided : — 



Ultrabasic 

 Dykes. 



fPyroxenites. 

 \Picrite. 



Intrusive Contact. 



Granites and fBiotite-Granites. 



Coarse -[Graphic Granite and other Pegmatites 

 Pegmatites. [Quartz- Veins. 



Intrusive Contact. 



^Biotite-Granites (including Porphyritic \ 

 I varieties). 

 I Pegmatites and Aplites. 

 c^uicti/iucn, -. py roxene _G ran jke and Quartz, 

 d Associated t> tv -4- a ■ 



Pyroxene-Lhonte feeries. 



Pyroxene-Granulites ? 



Intrusive Contact. 



Granulitic 

 Granites, 

 Pegmatites, 

 an 



Rocks. 



Gneisses 



and 



Associated 



Rocks. 



Biotite-Gneisses and 

 Gneissose Granites. 

 Hornblende Gneisses. 

 Amphibolites. 

 Garnetiferous Gneisses, 

 Granulites, and Eclogites. 



Intrusive Contact. 



Crystalline 

 Schists and 

 Limestones. 



Age unknown. 



Pre-Cretaceous 

 and Post-Middle 

 Pre-Cambrian. 



Middle 



Pre-Cambrian. 



Pb/U = 0-14-0-17. 



Pb/U = 0-21. 



Lower 

 Pre-Cambrian. 



^Quartz-Mica-Schists. 



Quartz-Magnetite-Schists. 



Hornblende-Garnet-Schists. 



Haematite-Schists. 



Forsterite Marbles, and other 

 ^Crystalline Limestones. 



The above correlations of certain groups of rocks with the 

 Lower and Middle Pre-Cambrian of other regions are based on 

 the determination of lead-uranium ratios of zircons derived from the 

 gneisses and granulitic granites respectively, the zircons having been 

 obtained by crushing and panning the rocks in the field. The 

 gneisses give a ratio of 0-21, comparable with a ratio of 0'24 obtained 

 for Canadian zircons of Laurentian age. The granulitic granites 

 give ratios of 0'14 to 1 7, comparable to those of radio-active 



