Great Dyke of Norite of Southern Bliodesia. 
3 
upon reaching the Great Dyke turn at right angles to their normal course 
to flow along the contact for short distances. In these places the 
pyroxenite shows a rude cross-jointing perpendicular to the edge, making 
horizontal pillars weathering into rude spheroids. The granite-gneiss is 
exposed as a smooth highly-inclined surface, indurated but not visibly 
recrystallized. In places along the edges the granite appears to have 
been considerably baked and now stands out as hills. This is well brought 
out by Wagner's mapping in the Belingwe district (see map, ibid.). 
Perhaps the most striking feature of the Great Dyke at Selukwe is the 
high degree of differentiation which the magma has undergone. 
The dominant rock types which make up the intrusion are felspar-rich 
norite, enstatitite, and enstatite peridotite. These outcrop in elongated 
•strips or wedges parallel to the edges of the intrusion, the less basic types, 
which are also the finest textured, forming the innermost strips. The less 
basic portion forms high ground ; of such nature is the large rectilinear 
range, the Selundi range, which forms throughout the area the central 
part of the Great Dyke. The least basic type of rock (granite), present as 
dykes and bosses in the marginal portions of the intrusion, stands out too 
in the form of walls and hummocks. 
The more basic parts of the intrusion, on the other hand, form low 
.ground, although this is not true of the Great Dyke in some other parts 
of Ehodesia. The two rivers above mentioned have almost invariably 
carved their present courses in the most basic rock types that are present. 
Certain quite subordinate types (websterite, granite, etc.) are apparently 
■distributed at random, and occur in small bosses and veins. 
At Selukwe the dominant types outcrop with a striking bilateral 
■symmetry. 
The two sides of the intrusion show several differences : there appears 
to be more peridotite and less felspathic pyroxenite on the east than there 
is on the west side, whilst websterites were only discovered on the 
w^est side. 
The rocks are entirely free from foliation, banding, or streakiness. 
They are fresh — generally ideally so — but the peridotites are to some 
extent serpentinized, and locally small portions of the enstatitite have gone 
to a bastite-like mineral. A stratiform appearance is to be observed in 
several places, one type of rock sharply demarcated from another will 
show a kind of bedding plane, the one type shelving under the other; such 
planes usually dip to the west. 
III. — Habit. 
The boundaries of the various types have not been mapped, but 
^ generalized plan of the distribution in a representative section 
