Chap. XIX.] CONCLUSIONS ON THE DIFFUSION OF GOLD. 
473 
which had been previously disseminated in those sediments through which 
the molten matter charged with the nobler metals had to pass. 
Whether the penetration of gold into the crust of the earth was confined 
to two epochs, as suggested by Mr. D. Forbes, must be worked out by close 
survey of many auriferous regions ; but of this we are certain, that, even 
after the first appearance of this metal, at each detrital accumulation, during 
Tertiary periods and even to our . own times, the gold has been washed 
down into hollows with the debris of the rocks in which it was formed. 
Hence the various ages of golden gravels or Drifts, whilst not a trace of 
the precious metal has ever been found in conglomerates or sandstones of 
Palaeozoic or Mesozoic age. No stronger proof than this can be given of 
the truth of my inference that gold was the last formed of the metals. 
Notwithstanding the preceding sketch, it would ill become any geologist 
to attempt to estimate, at this day, the amount of gold which remains un- 
detected in vast regions of the earth as yet unknown even to geographical 
travellers, still less to speculate upon the relative proportions of it in such 
countries. At the same time, the broad features of the case in all known 
lands may be appealed to, to check extravagant fears and apprehensions 
respecting an excessive production of the ore ; for we can trace the boun- 
daries, rude as they may be, of a metal ever destined to remain precious 
on account of those limits in position, breadth, and depth by which it is 
circumscribed in Nature's bank. Let it be borne in mind that no gold 
has ever been found in unaltered Secondary and Tertiary rocks, which 
occupy so large a portion of the surface, and that mines sunk to great 
depths in the veinstones of those rocks in which it does occur have hitherto, 
with rare exceptions, proved unremunerative. The only cases in which 
very deep mining in the solid matrix repays are chiefly those where the 
rocks are soft, or the price of labour low. Further, it has been ascer- 
tained that, whatever may have been the agency by which this impreg- 
nation was effected, the metal has been chiefly gathered together in rich 
veinstones towards their surface. It is by the abrasion and dispersion of 
these veinstones that the richest golden materials have been spread out, 
in limited patches, and generally near the bottom of basin-shaped accumu- 
lations of detritus. 
Now, as every heap of these broken auriferous materials in foreign lands 
has just as well-defined a base as each gravel-pit in our own country, it is 
quite certain that hollows or slopes so occupied, whether in California or 
Australia, must be dug out and exhausted in a greater or less period. In 
fact, all similar deposits in the Old or New World have had their gold 
abstracted from heaps the areas of which have been traced, and their 
bottoms reached; and the same result has, as I anticipated in the last 
edition of this work, been already to a great degree realized in Australia. 
Not proceeding beyond the evidences registered in the stone-book of 
Nature, it may therefore be affirmed that the period of such exhaustion 
