

ORE-FORMATIONS 261 
fissures in the rock, but here and there replacing the felspar, 
the form of which it retains. It is this constant passage of 
one type or form of epigenetic ore-formation into another 
that makes it impossible to separate them into well-defined 
or natural groups. 
3. Disseminations.—Ores are sometimes disseminated 
through a rock in such a way as to show that they are not 
original constitutents of the rock, but have been subsequently 
introduced ; for they occupy its minute pores, interstices, 
capillaries, and larger cavities. A good example of this kind 
of ore-formation is supplied by the copper-bearing sandstones 
and conglomerate which were formerly worked at Alderley 
Edge and Mottram St Andrews, near Macclesfield. Green 
hydrated copper carbonate (malachite) and the blue variety 
(azurite) are disseminated through the cementing material 
of the rocks, the constituent grains and pebbles being in this 
way coated with ore. Small quantities of ores of lead, 
manganese, iron, and cobalt occur in the same sandstones. 
The most notable examples, however, of such disseminations 
are the auriferous conglomerates of the Rand in Transvaal, 
S. Africa. The strata of sandstone in which the gold-bearing 
conglomerates occur at Witwatersrand, dip at a high angle 
(60° to 80°), but the inclination decreases as the beds are 
followed downwards. Gold occurs chiefly in the siliceous 
cementing material of the conglomerates, and is highly 
crystalline, appearing with sharp edges under the microscope. 
In this respect it differs from the gold of placers, much of 
which shows no trace of crystalline form. Associated with it 
are many secondary minerals, such as pyrite, marcasite, 
eEnlerite, talc, sericite, etc The strata contain no fossils, and 
were probably accumulated in alake. Subsequently the whole 
series of deposits were subjected to crustal movement, being 
tilted, compressed, fractured, and faulted, and then or at a later 
period were traversed by dyke-like intrusions of various 
igneous rocks (see Fig. 106). Concurrently with the crustal 
disturbance or with the igneous intrusions, siliceous and 
metalliferous solutions permeated the strata, making their way 
more readily through the conglomerates than the close-grained 
sandstones. Hence it is in the former that gold and crystallised 
minerals occur most abundantly. It is worthy of note, however, 

