On Mineral Vewns. 241 
For some time the question as to whether the’ contents of 
veins were of igneous or aqueous origin created much dis- 
cussion. This may now be considered as set at rest, the 
presence of undoubted aqueous deposits in veins in large 
proportions rendering it more than probable that all the 
contents are of this character. For example, there can be 
no question but that the quartz found in the veins of this 
colony is of aqueous origin. Bischof in the first instance, 
and Dr. Percy since, point out that although quartz may be 
formed artificially both by igneous and aqueous means, yet 
there is a difference in the specific gravity of the two 
minerals thus obtained, that of the igneous quartz being 2:1 
to 2:3, of the aqueous quartz 2°5 to 2°6. 
The quartz met with both in mineral veins and in tae 
granites has the specific gravity of aqueous quartz, and may 
therefore be safely considered as having been formed by 
aqueous action, and not by fire. This view is further sup- 
ported by the conditions attending these deposits, for on 
no other hypothesis yet advanced is it possible to account for 
the cases frequently met with of a change from pure quartz 
to quartz-rock, sandstone, and slate, so gradual as to render 
it impossible to draw a boundary-line between the different 
deposits, or for the surface veins and detached strings or 
leads of quartz, which are found throughout the lower silu- 
rian rocks, and have no connection with any fissure or 
opening through which the molten quartz could have been 
injected. 
Change is no doubt going on in mineral veins as it is in 
all other deposits, and the contents may have been fre- 
quently removed and replaced, as is indicated by the 
presence of pseudomorphic crystals of different minerals. 
The same agent that collected the minerals in the veins may 
re-arrange them, or even carry them away altogether, and it 
is in this way that cavities have probably been formed in 
the veins traversing the limestones, and where the walls 
were of hard rock, or where they had been left coated with 
solid mineral deposits, so as to allow the cavities to remain 
open, the space may have been filled in with the earthy matter 
and ore now found in some veins, or with the regular crys- 
talline deposits seen in others. During these changes, the 
corresponding vertical parallel ribs of minerals occasionally 
met with in veins may have been formed, and it is even 
possible that veins may have been partially or wholly obli- 
terated. Bischof mentions a case where a previous deposit 
