240 On Mineral Veins. 
irrespective of whether these joints were cleavage planes, 
divisional planes, or true faults. 7 
The lower silurians of this colony are highly metamor- 
phosed rocks, exhibiting the marked cleavage which usually 
attends the change from a compact crystalline structure to ~ 
a comparatively soft state, and in these rocks the quartz 
veins have generally formed on the cleavage planes ; but in 
the rare instances where they have formed on the divisional 
planes, they have usually been exceedingly productive. 
In the upper silurians, where the cleavage is imperfect and 
when present seldom extends over large areas, the veins 
generally form on the divisional planes, or on some fault or 
dyke. In consequence they are, as compared with the veins 
of the lower silurians, more apt to form rich deposits of sur- 
face quartz, which are soon exhausted, as they are not con- 
tinuous either in length or depth. Wherever they are 
of a more permanent character, it will be found that a 
change has occurred in the rocks bounding the vein similar 
to that observed in the lower silurians. 
Where these upper silurians are traversed by thin diorite 
dykes, the auriferous quartz veins usually form on the side 
of the dyke, in the same manner as the lead ore veins have 
formed on the side of the trap dykes in the island of Islay. 
In the wide diorite dykes the quartz veins frequently tra- 
verse the dyke itself, either as flat or highly inclined veins, 
or as irregular leads of quartz.  - 
The next point to be considered is the mode in which the ~ 
contents of the veins have been deposited, and the source 
from whence they have been derived :— 
In discussing this question occasional reference will be 
made to the phenomena observed in the veins of the car- 
boniferous or mountain limestone of the north of England. 
This formation consists of a series of nearly horizontal alter- 
nating beds of shales, sandstones, and limestones, and in the 
Derbyshire mining field, of alternate beds of limestone and 
basalt. In some of the limestones flat veins are formed, but 
generally the veins are nearly vertical, and thus afford an 
excellent opportunity for studying their changes of character 
as they traverse and are affected by the different beds of 
rock. Had more attention been given to the veins of this 
formation, a clue would have been obtained to the pheno- 
mena attending veins in the silurian rocks, and correct views 
would have been sooner promulgated as to the agents 
employed in collecting our metallic treasures. 
