Great Dyke of Norite of Southern Bhodesia. 
11 
composed of coarse blocks and rounded slabs ; such are seen near the 
Victoria road drift across the Little Tebekwe river (see Plate II, fig. 2). 
Elsewhere the rock disintegrates into coarse enstatite sand. Exception- 
ally it is observed to alter, notably along prominent joints and alongside 
the granitic veins for a few inches to two feet on both sides, to a pale 
pistachio-green bastite rock, which retains the form of the mineral and the 
structure of the original rock, but is soft (H = l-5), and has sp. gr. 2-74:. 
These and other characters suggest that the altered mineral is the variety 
of bastite called phastine. 
Usually the rock is coarser grained than are the other parts of the 
complex, but it varies from coarse, even-textured, granular, to fine, compact. 
Its colour is typically deep olive-green, but ranges from dark brown-green 
to grey-green. The grains of mineral composing the rock are equi- 
dimensional hypidiomorphic to idiomorphic. Some specimens have 
crystals ^-1 centimetre long arranged parallel to one another. In it 
segregation veins of fine- and coarse-grained enstatite occasionally 
occur. 
The rock consists essentially of rhombic pyroxene (enstatite), Plate III, 
fig. 4, with but minute proportions of the following accessory minerals : 
Basic felspar, monoclinic pyroxene (W. A. Humphrey, " The Geology 
North of Zeerust," Transv. Geol. Survey, Expl. of Sheet No. 9 (Marico), 
p. 25, refers to the presence of diallage in the pyroxenites of the Bushveld 
laccolite), brown mica, olivine (rarely), pyrite, chromite, and micro- 
pegmatitic quartz and orthoclase. By increase of the accessory plagio- 
clase and monoclinic pyroxene the rock grades respectively into the 
felspathic enstatitite type (Plate III, fig. 5) and to felspathic websterite 
(Plate III, fig. 6). 
The strongly pleochroic red-brown mica, the pyrite, and the basic 
felspar are fairly constant accessories ; the latter mineral tills the 
minute angular spaces between the enstatite crystals, thereby permitting 
the idiomorphism of the pyroxene. The enstatite in thin section is 
extremely pale-coloured, but exhibits faint pleochroism, pale pink to pale 
green. 
The specific gravity of this rock is 3*29 to 3-30. 
The Great Dyke enstatitite, but for the minute quantity of accessory 
minerals, is precisely similar to that of the Marico district, Transvaal, 
described by J. A. L. Henderson (Transvaal Norites, Gabbros, and 
Pyroxenites, London, Dulau & Co., 1898, pp. 38-41), who considers the 
rock to be " an extreme modification of a norite." It also appears to be 
exactly similar to the enstatite-pyroxenite of Yaalkop (Transvaal) described 
by H. Kynaston (Trans. Geol. Soc. S. Af., vol. viii., p. 57). 
Felspathic Enstatitite (Plate III, fig. 5). — This type is chiefly limited 
to the flanks of the Selundi range, but it also occurs along the extreme 
