396 Miscellanies. 
that the appearance of oxydation assumed by them during exposure to the 
atmosphere, is nothing more than the decomposition of the sulphuret into 
peroxyd of iron. That this decomposition would take place under these 
circumstances I may instance, in the case of an embankment of clay on 
the Croydon railway, where, accompanied by the formation of sulphate of 
lime, it occurred so extensively as to destroy part of the work. 
I have, on analysis, usually found deep clays to contain sulphuret of 
iron; and another proof of its existence in them may be adduced from 
the slaty clay usually accompanying the coal formation, where it is found 
so extensively as to be employed for the manufactures of alum and cop- 
peras.—London, Edinburgh and Dublin Phil. Mag., May, 1845, pp. 
440, 441. 
21. Sillimanite-—Dr. Thomson has lately obtained for the composition 
of this mineral the following: silica 45°65, alumina 49°50, protoxyd of 
iron 4'10—=99'25, which is the same as that given for Bucholzite—Phil. 
Mag., xxvi, 1845, p. 536. 
22. On the origin of Quartz and Metalliferous Veins; by Prof. 
Gustav Biscnor, of Bonn. (Jameson’s Jour., April, 1845, p. 344.)— 
The author opposes the theory of injection of the material constituting 
quartz veins in a state of fusion, and advocates the view of their origi- 
nating from aqueous solutions. The article is worthy of being cited en- 
tire. The following are interesting facts with regard to the prevalence of 
silica in solution in cold water :— 
There is scarcely any water, whether it be spring or river water, which 
does not contain silica in solution, though frequently in very small quan- 
tities. Should such water penetrate through the narrowest cleft, there is 
the possibility that more or less of the dissolved silica may be deposited 
init. It is true, that such a deposition supposes that the water, either, 
being hot, cools during circulation in the cleft, or evaporates; or that other 
substances maintaining the silica in solution are precipitated ; but we must 
not overlook other circumstances from which this precipitation may arise. 
Very many phenomena show that there exists a peculiar affinity between 
silica and organic substances or organic remains. As an example, I may 
mention that in the wooden piles of 'Trajan’s Bridge, near Vienna, quartz 
concretions—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. 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. 
