120 
Geology and Physical Geography: 
the next consideration is the method by which the quartz-veins 
were formed into fissures, and the associated minerals conveyed 
and deposited in or with the quartz. In the first place it is known 
that silica, the principal, or nearly the sole constituent of quartz, 
is capable under certain conditions of being hold in solution and of 
being deposited from that state in various forms, crystalline, concre- 
tionary, laminar, &c. Of several theories once entertained as to 
the formation of quartz and other mineral veins or lodes, one was 
that they were deposits, in the fissures, of minerals held in solution 
by the sea during some period when the country was submerged 
This is untenable, for the simple reason, among many others, that 
under such conditions the natural results would be the speedy fill- 
ing up of the fissures by mechanically-formed sediments of mud 
and detritus, of which we find no evidenco whatever. Another 
theory was that the quartz was injected in a molten state like an 
igneous dyke. This is refuted by nu overwhelming mass of 
evidence, some of which also bears on the first-mentioned theory. 
The various structures exhibited by quartz, whether crystalline 
amorphous, or laminated, are not those which it could be expected 
to assume on cooling from a state of fusion, whether quickly or 
slowly, or at whatever depth below the surface. 
The walls of quartz-veins show not the least indication of having 
been subjected to dry heat emanating from such voius. In the case 
of quartz-veiris traversing dykes, we see plainly that the latter 
whatever their origin, were injected first, and that in shrinkao-e- 
cracks and other fissures which formed in or along them during°or 
subsequent to their consolidation, the quartz has since been formed 
in lodes, veins, strings, or bunches, in a manner wholly irrecon- 
cilable with the idea that it was ever in a molten condition ; and 
further, we find frequent instances of bunches or small veins or 
patches of quartz occupying hollows or cracks remote or totally 
isolated from large veins, and unaccompanied by any indication of 
having attained that position by beiug injected in a molten state.. 
Such quartz-veins are occasionally to be met with in the Upper 
Pakeozoic (? Upper Devonian) rocks of the Avon, under conditions 
which preclude all possibility of their being of Plutonic origin. 
Dismissing the above theorios, we can turn to a third, which 
affords the most probable explanation. It is that the quartz veins 
are the results of the segregation of silica from the surrounding 
rocks themselves, or the subjacent Plutonic masses, and its coii^ 
veyauce by percolation — probably hydrothermal — waters into the 
fissures, where it was precipitated, according to local conditions 
in the various forms assumed by quartz, much in the same way as 
we see calcareou.9 veins in actual course of formation in cracks 
and fissures in limestones, and even in rocks containing but a 
small proportion of lime. If to this action was added, as it is 
reasonable to suppose, that of heated mineral waters, steam, and 
