G4 
THE GEOLOGIST. 
difficulty, as Dr. Genth truly says, " presents itself on inquiring into 
the nature of the solvent." The doctor believes the noble metal was 
dissolved as terchloride of gold : — " If we remember that the decom- 
position of pyrites produces sulphm-ic acid, which, in the presence of 
the never wanting chloride of sodiimi and a higher oxide of manga- 
nese, may liberate smaU quantities of chlorine, the most powerful 
solvent of gold, we have at least a very plausible explanation." 
Dr. Genth's doctrine is that the gold both of veins and alluvial de- 
posits is derived from the adjoining rocks, where it exists probably 
in a highly disseminated state ; from these rocks it is removed in a 
state of solution, and precipitated in concentrated deposits in the 
veins and among the alluvial debris. The rock in which the metal 
most frequently occurs is diorite or greenstone [composed of ortho- 
clase felspar and hornblende]. How it originally came into this 
rock is not a qiiestion entered upon ; the main point being to prove 
that the gold of alluvial deposits is not derived, as is usually sup- 
posed, from the destruction of pre-existing quartz-veins, the fact 
being that they are both equally derivative, the original source being 
the neighbouring rock. 
There are many considerations suggested by the mode of occurrence 
of gold which weigh strongly in favour of such a suggestion. In the 
first place, gold is continually found in alluvial deposits, in consider- 
able sized nuffgets, in districts where no veins are found. In this 
case the usual theory is that the original upper surfaces, which were 
rich in auriferous veins, have been removed by denudation, and that 
the alluvial gold is the remnant of these. But this has the disad- 
vantage of assuming an hj^pothesis, for which no good reason can be 
given, that there is some peculiar law which limits the original pro- 
duction of gold to the surface, and which is held, with a strange in- 
consistency, by the same men who fully recognize the more than 
probability of other metals being derived from beneath. Assuming 
that the gold is derived from the rock by solution, and that the veins 
are mere depositories, we have at once at least a plausible reason for 
the general occurrence of gold only near the surface. It is that the 
solvent of that metal is somehow or other essentially connected with 
the atmosphere ; so that although gold may exist at all depths, 
scattered, highly disseminated through certain igneous rocks, it is 
