346 Professor Bischof on the Origin of 
in solution are precipitated ; but we must not overlook other 
circumstances from which this precipitation may arise. Very 
many phenomena shew that there exists a peculiar affinity 
between sélica and organic substances or organic remains. AS 
an example, I may mention that, in the wooden piles of Tra- 
jan’s Bridge, near Vienna, quartz coneretions—agates even 
of half an inch in thickness—have been found ;* and that, 
according to observations of Glocker, it is only on a lichen 
that Hyalite is formed on the Serpentine of the Zobtenberg.t 
If, now, in the above instances, the wood of the bridge pile 
has induced a precipitation of silica from an extremely dilute 
solution, such as the water of the Danube presents—if, in 
like manner, a lichen has occasioned such a precipitation, 
from, probably, equally dilute solutions, then it is easy to 
understand, that organic remains in a Neptunian rock, as in 
clay-slate, may likewise effect a precipitation of silica. 
It may be said, in opposition, that the supposed effect of 
organic remains must cease in the rock as soon as the thin- 
nest covering of precipitated silica has been formed ; but it 
is known, that, as soon as a deposition of a dissolved sub- 
stance has begun from whatever cause, it goes on, although 
this cause may no longer be in action. I am, however, far 
from alleging, that the presence of organic remains in geo- 
logical formations, has always induced the deposition of 
silica in quartz veins. If such were the case, this effect 
could only be considered as acting in quartz veins in Nep- 
tunian formations, and would have no application to such 
veins in the crystalline formations. 
It becomes, however, necessary to inquire into the causes 
by which deposits of silica from watery solutions may have 
been effected. Is it not sufficient to refer to the numberless 
quartzy formations which must unquestionably have had 
their origin in the wet way ? Can we, in the frequent silici- 
fication of organic substances—of wood in wood-opal, for 
* Breislak’s Geologie, Bd. ii., p. 492. 
+t Verhandlungen, der K. L. C. Akad. d. Naturforscher, Ba. Xiv, 
Abth. ii., p. 545. Compare also Von Buch on the Silicification of Organie 
Substances in the Abhandlungen, der K. Acad. d. W. zu. Berlin, 1828, 
p. 48, “ Where” says Von Buch, “ there is no organic substance, there 
is never silicification to be found.” 
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