22 
Transactions of the Boyal Society of South Africa, 
The position of the granite in the table is very uncertain. 
All the types mentioned are present in the Great Dyke, but the 
primary (enstatite-rich olivine norite) and secondary (norite and enstatite 
picrite) magmas are only represented by small amounts which may 
perhaps be regarded as residua. These latter, together with certain of the 
secondary subordinate differentiations (websterite and granite) occur in 
the form of small bosses and veins. The dominant secondary differen- 
tiations (enstatitite and felspar-rich norite and gabbro) occur in large 
parallel-sided bodies extending, generally speaking, from end to end of 
that portion of the Great Dyke examined. The subordinate secondary 
differentiations, harzburgite and felspathic enstatitite, occur in elongated 
wedges. 
Broadly speaking, therefore, the parent magma has separated into 
two dominant types in about equal proportions and two subordinate 
ones as follows : — 
Dominant. 
Bytownite-diopside-enstatite rock containing 30 per cent, 
pyroxenes and 70 per cent, felspar. 
Enstatite rock containing 90 per cent, pyroxenes and 10 per 
cent, felspar. 
Subordinate. 
Enstatite-bytownite rock containing 80 per cent, pyroxenes and 
20 per cent, felspar. 
Olivine-enstatite rock containing 50 per cent, pyroxenes and 
less than 5 per cent, felspar. 
If these be considered from mineralogical data to be complimentary and 
co-ordinate derivatives of the original magma, the latter would seem to 
have been of norite composition — an enstatite-rich olivine norite, that is, 
a basic magma rather than an ultra-basic one, which the large amount of 
ferromagnesian minerals would at first sight lead one to infer. But the 
complex is apparently abnormal mineralogically, being notably rich in 
rhombic pyroxene and somewhat deficient in felspar. 
Chemically the complex would seem to be a fairly normal basic one 
so far as its roughly calculated silica content (about 51 per cent. SiOa) 
is concerned, but it would seem to be unusually rich in magnesia, normal 
to high in lime, but deficient in alumina and poor in iron. This is 
expressed in the following table, which is an attempt to indicate the 
relative amounts of principal minerals in the complex : — 
Excessive amount of rhombic pyroxene \ , , . , ... 
o u ^ n L r • Slightly high silica, 
Somewhat small amount or olivine Y ^. ^ j^r^^^-^ggjg^ 
Moderate to rather small amount of bytownite J ' ° 
