474 
SILUKIA. 
[Chap. XIX. 
in each country (for the deposits of detritus or fields of diggings are much 
shallower in some tracts than in others) will, in great measure, depend 
on the numbers and activity of the workmen employed in each locality. 
Numerous hands, used with Anglo-Saxon energy, in California and Aus- 
tralia may in a quarter of a century accomplish results which could be 
attained only in many centuries by a scanty and lazy indigenous popula- 
tion ; and thus the present large flow of gold into Europe from such tracts 
must, in my opinion, diminish as soon as the auriferous detritus of certain 
tracts has been well sifted and the richest or upper parts of the veinstones 
have been worked out. 
In defining the general character of the most productive auriferous 
rocks, the geologist must, however, necessarily admit a number of excep- 
tions to any prevailing rule ; for, whilst the chemist, as before said, has 
recently detected minute traces of gold in lead- and copper-ores, the re- 
searches of the practical miner have taught us that in any auriferous 
region where certain quartzose lodes are surcharged with ores of iron, 
particularly the oxides and sulphurets, some amount of gold will probably 
be found. Again, the diffusion or dissemination of small particles of gold 
throughout the body of various igneous rocks, and also, to some extent, in 
altered Secondary rocks of aqueous origin, is, as before said, clearly ascer- 
tained. Humboldt, indeed, asserted long since, that in Guiana, "gold, 
like tin, is sometimes disseminated in an almost imperceptible manner in 
granitic rocks, without the ramification or interlacing of any small veins"*; 
and we now know that this phenomenon is general along the chain of the 
Andes. In Mexico the gold-mine of Guadalupe y Calvo, before alluded to 
(p. 460, note), was in porphyry. In Australia (district of Braidwood, and 
other places south of Sydney), a peculiar variety of felspathic granite is 
described as being permeated by small particles of gold ; and David Eorbes 
has shown, as above stated, that it is present in diorites as well as in the 
granites of South America. In Siberia, Hoffmann f has spoken of its dis- 
tribution in such minute quantities in clay-slate that it was only by 
pounding up large lumps of the rock that any perceptible quantity could be 
extracted ; whilst I have pointed out its occurrence in the granite, syenite, 
greenstones, and altered Silurian rocks of the Ural Mountains (p. 454) . 
In all regions, therefore, where auriferous rocks occur, we may find gold 
either in the coarse debris of Tertiary and Quaternary ages, or in more 
modern alluvia. Felspar and quartz being the chief component parts of the 
veinstones, we can easily imagine how their former destruction on a large 
scale would leave as a residue large heaps of that pipe-clay (the decom- 
posed felspar), or those gritty pebbles (the abraded quartz), which, with the 
accompanying ores of iron (particularly the black magnetic oxide), are so 
frequently the gold-bearing matrices in the Drift of auriferous countries. 
* Voyages, vol. ii. p. 238. 
t Keiscnach dem Goldw'aschen Ost-Sibiriens: St.-Petersburg, 1847. 
