92 
Rex T. Prider. 
They are mostly coarse grained types (average grain size 3 mm. or more) 
and have the same granoblastic structures as noted above, the main differences 
to the Upper Quartzite being the almost constant presence of felspar in the 
Lower Quartzites and its absence in the Upper Quartzite and the fact that tlie 
chrome muscovite in the Lower Quartzites is generally set between the quartz 
grains while in the Upper Quartzite it occurs as inclusions in the cpiartz. 
Most hand specimens are very coarse sugary rocks with bedding planes 
sparsely coated with pale greenish chrome-muscovite. White turbid felspar 
is a common constituent, but is never present in amounts exceeding five per 
cent. It is usually a microcline, but rare gi*ains of oligoclase and orthoclase 
are present. The felspar grains are much smaller than the quartz, reaching 
a maximum of about 0 * 5 mm. diameter — ^they are usually somewhat rounded, 
slightly turbid inclusions in the quartz, but the larger grains show a tendency 
to idiomorphism (figure 4A). 
Pig. 4. 
A. Pelsj>athic quartzite, showing coarse granoblastic structure 
and tendency of larger microclines to assert their crystalline 
form. 
B. ‘‘Heavy mineral’^ band in lower quartzite. 
Minor detrital minerals occur sparsely in these lower quartzites — the most 
common species are rounded grains of zircon, magnetite and rutile. Pyrite 
in rounded grains with a thick rim of limonite is found occasionally— there 
can be little doubt, in view of the rounding of the grains, that the pyrite is 
an autochthonous constituent of the rock. The segregation of these ‘‘ heavy ” 
minerals into bands (figure 4B) has been previously noted. 
(c) The Origin of the Quartzites . — The (piartzites, then, are rocks of simple 
composition consisting almost entirely of quartz (95 per cent + ). There can 
be little doubt that they were originally remarkably pure sands which have been 
completely recrystallised in the sillimanite zone. The coarse granoblastic 
structures, which have completely obliterated any sign of original clastic 
structures, testify to the high grade of metamorphism to wliich tliey have been 
subjected. In the absence of any foreign material, the only change in these 
rocks is the complete recrystallisation of the quartz. Hall and du Toit (1923. 
p. 77) in describing the very coarse quartzites at the base of the Bushveld 
