350 Professor Bischof on the Origin of 
though only one, we find also in metalliferous veins a more 
frequently repeated alternation of vein stuffs. If these sur- 
face deposits shew, incontestibly, that the same spring may 
change the nature of its deposit in the course of time, we 
can, from this, conclude as to the possibility, at least, that also 
the alternating members of the vein, in metalliferous veins, 
may have had the same origin. 
If we recur to the question of how the silica in quartz 
veins may have been precipitated from watery solutions, 
we cannot consentaneously suppose that this deposit has 
been a consequence of a decrease of temperature of the dis- 
solving medium, or of its vaporization during the circulation 
of the water through the clefts ; and as the assumption that 
organic remains in the adjacent rock have played a part, is 
only admissible for quartz veins in Neptunian formations, 
there remain still certain difficulties to solve. These de- 
crease very much, however, if we take into consideration a 
fact that the regularity appears very frequently disturbed, and even 
adduces many examples of this. 
The question is, whether order is the rule and confusion the excep- 
tion, or whether the former is only fortuitous? As the formation of 
veins is a process occupying a great space of time, and as, after it had 
already begun, the fissures again widened out and the new fissure took, 
here parts of the adjacent rock, there parts of the vein stuff, according 
to the state of adhesion and cohesion between the rock and the vein 
stuff already formed, and of this latter with itself; it must have hap- 
pened that the more recent vein stuffs now gained the interior,—now 
were deposited on the walls and on the fragments torn from the adja- 
cent rock, To this has to be added, that there not unfrequently took 
place an exchange between the older and newer members of the vein, 
by, the; former being taken up by the solvent medium, and the latter 
being deposited and taking the place of the former. It might thus also 
so happen, that (if we suppose the introduction of the vein matrices in 
the: wet way,). the same fluid, containing several vein-stuffs in solution, 
according as in one part it came in contact with the adjacent rock, and 
in another with older members of the vein, deposited by exchange one 
substance here, another there, just.as.a fluid, would do if we added first 
one and then another re-agent. All these causes might) produce the 
most. various disturbances. in the formation. of the vein-stufls, so that 
regularity only appears in those instances in which these disturbing 
causes have not acted. I have enlarged upon these circumstances in 
the sequel. 
