On Mineral Veins. 233 
to allow these fissures to remain open. There is no evidence, 
_ however, to support such an assumption, and all analogy 
would lead to the contrary belief. But even supposing 
that the rocks had at some time been so hard as to allow 
of open fissures, penetrating to a great depth, this theory 
will not account for the formation of the small detached leads 
and bunches of quartz found scattered throughout the silurian 
rocks wherever these rocks have been much decomposed, nor 
for the shallow gash or wedge-shaped veins which do not 
penetrate far from the surface. In these cases there could 
be no circulation of currents of water, for the quartz leads 
only extend afew feet, and the gash veins are closed before 
_ they reach the deep source from whence the contents of the 
veins are, it is assumed, derived. 
The conditions under which riders (z.e. detached masses of 
the bounding rocks enclosed in the vein) occur in veins are 
also incompatible with the open fissure theory. Fig. 6 is a 
section of a lead vein in the limestone rock, where the dark 
lines represent the ore in the vein, and in the strings or 
joints, ¢ ee, connected with it. It will be seen that the 
Fig. 6.—Vertical Section. 
R 
