A. R. Andrew—The Dolgelley Gold-belt. 269 
proceeded to any great depth it is possible that the whole of the lodes 
so far mined may be in part enriched by secondary enrichment. 
GENESIS OF THE AURIFEROUS VEINS. 
The general idea of the genesis of auriferous veins is that they 
were filled by waters carrying gold and other minerals in solution ; 
these waters rose from below through fissures and deposited their 
metallic contents in those fissures owing to— 
(1) Decrease of temperature, which caused the solutions to become 
supersaturated and to deposit part of their contained minerals. 
(2) Decrease of pressure which acted similarly. 
(3) Reactions between the ascending waters and other solutions 
which entered the fissure along its course; a common precipitant for 
gold, which is also a common substance underground, is ferrous sulphate, 
and another is carbonaceous matter. 
(4) Reactions between the ascending waters and the solid material 
forming the walls of the lodes. 
Subsequently the lode so formed was subjected to a process of 
secondary enrichment, which, by solution and re-deposition of the 
gold content, caused a relative enrichment of the upper portions of 
the lode. 
Applying these general principles to the Dolgelley Gold-belt, there 
is to be remarked, on the one hand the total absence below the Clogau 
horizon of any black or carbonaceous bed, and on the other the 
total absence of any auriferous vein below that horizon. It appears 
that the solutions rising in fissures through the Lower Cambrian rocks 
were affected only by decrease of temperature and pressure, which 
were not, however, sufficient to cause any important deposition of 
gold. At the horizon of the Clogau Beds, however, solutions entered 
from the wall-rocks, containing ferrous sulphate and organic matter, 
in both of which the Clogau Beds are rich; interactions between the 
two solutions caused a precipitation of the gold. As the solutions 
passed upwards into the Maentwrog Beds, more and similar solutions 
would enter, and gold would be deposited there also. The greatest. 
amount of gold, however, would probably be deposited at the Clogaw 
horizon. This theory tallies with what we know of the occurrence 
of the gold in the field; far and away the bulk of the gold has. 
been obtained from the two lodes whose outcrops are in the black 
Clogau Beds. 
The most ready channels for solutions to enter would be through 
cross-fractures, and at these points we might expect to find greater 
deposition of the gold. So far, we only know of this having occurred 
im one instance at the St. David’s lode and one at the Panorama lode; 
but the paucity of examples may be due to deficiency of observation, 
for no one appears to have been on the look out for cross-courses and 
their possible effects. 
The almost constant association of the auriferous beds with intrusive 
greenstones means only, I think, that the heat of the magma gave the 
percolating waters during the period of the intrusion a greater chemical 
activity, and rendered them capable of dissolving out and holding in 
solution more substances with which they came in contact. 
