Reviews—Southern Rhodesia. 229 
are several phases and dyke-like modifications. An even-grained 
dolerite, with chilled margins, forms sills, dykes, and stocks among 
both the schists and the granitic rocks, and is considered to be of 
the same age as the dolerite of the Karroo Beds, or possibly, as that 
of the ‘‘ great dyke’’ of Waterberg age. Outhers of Kalahari” 
Ironstone (Upper Cretaceous) are mapped on the higher land near 
the Salisbury Water Works. 
The greenstone schists were originally volcanic rocks and range 
from intermediate to ultra-basic, the commonest being a fine-grained 
epidiorite derived from dolerite or basalt. The ultra-basic types 
are now represented by serpentine, and talc- and tremolite-schists. 
The present structure and mineralogical constitution is due to 
regional metamorphism accompanied by high temperature without 
much shear. Evidence of contact alteration by the granite is con- 
fined to minor changes quite near the mass. The sediments which 
are folded in with the greenstones comprise banded ironstone, 
black phyllite, and limestone. The banded ironstone is a quartzite 
with layers of varying thickness containing abundant magnetite, 
originally a ferruginous sandstone. It has been worked for 1ron- 
ore by the natives. The phyllite is dark chiastolite slate grading 
into mica schist. The limestones are coarsely crystalline and contain 
green amphibole as a metamorphic product. 
The granitic rocks show a great variation in form and composition, 
but they are all of about the same age. They occur as bathyliths, 
stocks and vein-like intrusions or irregular bodies of aplite and 
pegmatite, the last being greisenized and stanniferous. Oligoclase 
is the principal felspar. 
A variable series of felsites, quartz-sericite-schists, and ultra- 
acid intrusives of the same age as, or in part penetrating, the granite, 
occur as irregular bodies or in dyke-like forms. They possess 
a dominant cleavage in the direction of flow-structure, due to the 
orientation of the biotite and sericitic mica. 
The Kalahari Ironstone is an eolian deposit, consisting largely 
of polished quartz-grains, and near the base of amygdules of 
chalcedony from Karroo basalts. The iron-ore near the base is in 
the form of sandy nodules surrounded by hard crusts of limonite. 
It is thought that the iron may have been derived from wind-borne 
chalybite which has been deposited among the sand and 
subsequently carried down and deposited as limonite. 
The gold reefs are classified as ‘‘ Schist Reefs” and ‘‘Quartz Reefs’. 
The latter are fissure veins, either (a) crossing the Basement Schists, 
as in the case of the Perseverance Reef, or (b) roughly parallel with 
the planes of cleavage. Gold in the quartz veins is seen to occupy 
rusty cavities formerly containing auriferous pyrites and possibly 
arsenical pyrites. Below the zone of oxidized ores the gold occurs 
in auriferous pyrites, and is termed “‘ refractory ore’’, owing to its 
not being free milling. 
The “ Schist Reefs’ are more abundant and are replacements or 
