Great Dylcc of Norite of Southern Bhodesia. 
7 
gravity upon the products, or by the injection of one differentiate into 
the other. 
It must be conceded by most petrologists that the layers of chromite 
described below are the result of differentiation in place and segregation 
of matter contained in the surrounding rocks. If layers of chromite rock 
are thus produced and constituted gently dipping layers alternating with 
peridotites, why not peridotites and pyroxenites too ? 
It seems to the author that such a process might have given rise to the 
condition admirably represented by Wagner {loc. cit., figure, p. 50), where 
a series of alternating layers of peridotite and pyroxenite are overlaid by a 
"bed" of harzburgite. The draining off of the pyroxenes from a harz- 
burgite magma would produce peridotite and pyroxenite, which it might 
be expected would settle into a position perpendicular to the sides of the 
intrusion. 
There is no evidence of uplift of the rocks or disturbance of the 
foliation alongside the intrusion in the Selukwe portion such as would 
be expected if the intrusion were a laccolite. 
The parallelism of the " Um Vimeela " Norite Dyke of Wagner {loc. 
cit., p. 52) (inferred to be genetically related to the Great Dyke) not only 
in the Belingwe area but northwards in the Selukwe and Gwelo areas is a 
fact of some significance to be noted here. 
IV. — Peteography. 
The petrology of the Great Dyke at Selukwe is extremely interesting. 
The examination of the intrusion, however, both in the field and laboratory, 
has not been carried out in sufficient detail to permit the settlement of 
many points. The field evidence is often disappointingly meagre. 
It has already been stated that the intrusion is made up of several very 
well-marked types of rock which are abnormal in composition. The 
differentiation of the parent magma has taken place to a remarkably 
complete degree. Thus quartz veins form one extreme and chromite 
seams the other. 
The differentiation resembles that of the Bushveld complex of the 
Transvaal in many respects (see Molengraaff, Bidl. de la Soc. Geol. de 
France, 4me, serie, tome 1, p. 48, 1901 ; F. H. Hatch, Trans. Geol. Soc. 
S. Af., vol. vii., p. 1, 1904, and Hatch and Corstorphine, The Geology 
of South Africa, 2nd ed., Macmillan, 1909, pp. 208-18). 
Considering the Selukwe part of the intrusion as exposed en masse the 
minerals are as follows : A largely preponderant amount of enstatite, 
considerably less but next in abundance being felspar (usually bytownite), 
a fair to somewhat small amount of olivine and of monoclinic pyroxene 
