21 
It is an important consideration to enquire under what con¬ 
ditions are the diamonds found in the old and well-known and 
studied localities ? 
These localities are India, Borneo, Sumatra, Australia, the 
Urals, Algiers, California, and N. Carolina and Georgia. In all 
these places they occur in alluvial gravel derived from more or 
less distant mountains. The general inference is, however, 
that this gem occurs in regions wUich afford a granular quartz 
rock called itacolumite, which belongs to the talcose series, 
owing its lamination to a little talc or mica. This rock is 
found in the mines in Brazil, the Urals, Georgia, and N. 
Carolina. It has also been detected in a species of conglo¬ 
merate composed of rounded silicious pebbles, quartz, chalce¬ 
dony, &c., cemented by a kind of ferruginous clay. But they 
are usually found washed out from the soil. In Brazil only 
have they been found in their native beds, and in two different 
deposits, one called gargulho, consisting of broken quartz 
covered by a thin bed of sand or earth; and a second cascalho, of 
rolled quartz pebbles united by a ferruginous clay resting 
usually on talcose, micaceous or schistose clay. These with 
some accompanying limestone bands evidently represent in 
altered conditions old sandstones, clays, shell-beds, &c., such as 
constitute a formation of marine or ffuvio-marine deposits. The 
diamonds which occur in these Brazilian schists are always re¬ 
garded as being the results of some of the changes which have 
affected the strata, and they may represent the carbon of the 
old carbonaceous deposits separated from the other constitu¬ 
ents either within the original mass of the strata or sublimed, 
and we have evidence of heat in the metamorphism, through 
fissures, from probable sources of carbon in Africa. 
There is then a large number of facts, which go to show that, 
as in other countries, diamonds are most usually found in trans¬ 
ported materials, and the great question which the seekers 
desire to solve is to trace their origin and learn whether it 
would not be more profitable to look for them in that matrix 
material. 
A suggestion has been made by Dr. Rubidge and others, that 
large areas of this part of Africa have been covered with alluvium 
