4 Transactions of the Boyal Society of South Africa. 
(probably no two sections are alike) at Selukwe may be represented 
thus : — 
Fig 1. — Diagrammatic Plan op Distribution of Rock Types in Great Dyke 
at Selukwe. 
N = Felspar-rich norite. X = Enstatitite. FX = Felspathic enstatitite. P = Peridotite. 
W = Websterite. G^ = Granite and pegmatite. Gf = Granite-gneiss country. 
Such a plan admits of at least two interpretations as to the habit of the- 
intrasion. An intrusion made up of rocks outcropping in this manner may 
be a dyke or a sill ; if the former, the distribution of the rocks within the 
intrusion may be represented thus : — 
Vs/ UmteVekwe River oeLuTidi Ranae L Telrekwe River £^ 
Fig. 2.— Diagrammatic Section across Great Dyke at Selukwe. 
Both explanations have been offered, and a third theory is suggested 
by P. A. Wagner (see below) that the intrusion may be a laccolite. 
F. P. Mennell states {loc. cit., p. 372) that the intrusion "is almost 
certainly a gently inchned sheet injected along a line of weakness, which 
is probably a thrust-plane." 
The present writer (loc. cit., p. 13) suggested in describing the Selukwe 
section of the intrusion that it was a dyke. 
P. A. Wagner in his instructive paper describes {loc. cit., p. 49) and 
gives a figure of a very clear section of a bedded arrangement of alternate 
layers of the peridotite and pyroxenite gently dipping to the south-south^ 
east ; such structures occur in several sections to the west of the centre Ime 
