2 
lo8 
ECONOMIC GEOLOGY 
The pre-Palzozoic ore deposits of Northern Rhodesia in- 
clude at the Rhodesian Broken Hill Mine a large body of 
complex lead-zinc ore deposited at the intersection of two 
fissures traversing dolomite and dolomitic schists. There 
are no adjacent igneous rocks, and the ore must be due to 
solutions rising from below ; like many pre-Palzozoic ores 
their composition is varied, including lead, zine, silver, 
vanadium, arsenic, and phosphorus. The bulk of the ore 
was deposited by replacement in limestone, and that now 
being mined is a mass of secondary carbonates, sulphates 
of lead, and silicate of zinc due to descending solutions. The 
mine was discovered in 1902, but the difficulties in treating 
its complex ore and the distance of 1330 miles from its port 
at Beira delayed development until after 1916. The ores 
will doubtless pass downward into primary sulphides, which 
have already been found in bore holes at depths of between 
150 and 400 feet. 
SEDIMENTARY OrES—Lead minerals being soft and soluble 
are comparatively rare in alluvial deposits, but bedded ores of 
sedimentary origin are found at Commern, N. of the Eifel 
in Western Germany. A bed of white Lower Trias sandstone 
contains concretions up to a quarter of an inch in diameter, of 
quartz grains cemented by galena or cerussite; the richer 
ore yielded 2 per cent. of lead, but was profitably mined as 
the concretions were easily concentrated. The lead was prob- 
ably present in small alluvial grains that have been dissolved 
and redeposited as concretions. A conglomerate with a 
cement of pyrite and galena in the Upper Trias at St. Sebas- 
tian in the Department of Gard in France, probably also 
obtained its lead from alluvial grains; its galena cement 
is analogous to the copper cement in the conglomerates of 
Michisan. 
SOURCE OF THE LEAD IN Loprs 
The lead ores illustrate the independence of lodes of the 
country rock. In many areas as in Great Britain, the 
Linares region of Spain, and the Rocky Mountains, the ordin- 
ary lodes of lead are strikingly similar in composition and 
essential features whether they occur in granite, slate, sand- 
LS. J. Speak, Mig. Mag., xxi, 1919, pp. 203-9.