738 Ro A ROPE NROSE. IR: 
ference being that the lower series is composed largely of slates, with 
quartzites and rarely some thin conglomerates, while the upper series 
is composed largely of quartzites, with prominent conglomerates and 
some little slate. Sheets of diabase are interbedded with both the 
upper and lower series, and diabase dikes cutting the strata t: ansversely 
are of frequent occurrence (Fig. 1). The whole Witwatersrand system 
is much faulted and broken, especially at the east and west ends 
of the district, making estimates of its thickness in some p’aces often 
very uncertain. It is known to vary considerably in thickness, how- 
ever, in different parts of the region, being much thicker in the  estern 
part of the Witwatersrand than in the eastern part. In the central 
part of the district the upper and lower series of the system are each 
about ten thousand feet or possibly somewhat more in thickness, giving 
an aggregate thickness for the whole system of approximately twenty 
thousand feet.t To the east they are thinner and to the west thicker. 
"he Upper Witwatersrand series has been divided by the South 
African geologists into several different formations, each one consist- 
ing largely of quartzite but marked by more or less prominent con- 
glomerate beds. These conglomerates are locally known among the 
and the term has been retained in some of the 
local geological nomenclature. The divisions of the Upper Witwaters- 
rand made by Hatch and Corstorphine? are, in a descending order, 
the Elsburg series, the Kimberley series, the Bird Reef series, the 
Livingstone Reef series, and the Main Reef series. The gold of the 
Witwatersrand mines occurs mostly in the Main Reef series, lying as 
it does at the base of the Upper Witwatersrand and just above the 
Lower Witwatersrand series. Small quantities of gold have been 
found elsewhere in the Upper Witwatersrand series, bu: rarely in 
paying quantities, though in some places extensive work has been 
done in search of it. Small quantities of gold have also been found 
in the Transvaal in other conglomerates than those of the Witwaters- 
rand system, but have not become of great importance. This is 
miners as “‘reefs,” 
t Hatch and Corstorphine, op. cit., pp. 108, 125. 
2 The Geology of South Africa, p. 122. Some doubt is expressed by these writers 
as to whether the Elsburg series belongs to the Upper Witwatersrand or to the overlying 
Ventersdorp system. This matter, however, is not of great importance in the present 
discussion, as almost all the profitable mines on the Witwatersrand are in the Main Reef 
series. 
