Quartz and Metalliferous Veins. 347 
example—entertain, even remotely, the idea of formation by 
heat. Ehrenberg found, as he told me, a Vermetus, an inch 
long, in fire- opal. 
The deposit of silica in quartz veins can be conceived to 
have taken place by ¢wo modes. Hither springs rose up in the 
vein cleft, from which it was deposited, or water, containing 
silicie acid, penetrated from the adjacent rocks into the vein 
cleft. Both processes occur at the present day, although 
deposits of pure silica from springs are rarities. Examples, 
too, are not wanting of the locality of the discharge of the 
spring changing, or of the discharge completely ceasing. 
Both cases, doubtless, arise most frequently from the under- 
ground channels being stopped by deposits. 
I have myself observed the complete cessation of a mineral 
spring. About twelve years since, there flowed, close by the 
Laacher-See, near the old abbey, a pretty abundant spring, 
which, judging from its enclosure, was used in former times 
by the inhabitants of the abbey. I visited this spring several 
times, as it excited my curiosity, being the only one of all 
the numerous springs in the neighbourhood of the Laacher- 
See, which shewed not even a trace of iron. It was a pure 
seltzer, containing, principally. bicarbonate of lime and mag- 
nesia. On going to see this spring, some years later, I 
found it stopped. There are also, very frequently, to be 
found in this neighbourhood, even considerable beds of éron- 
ochre, which indubitably have been deposited from irony 
springs, although such no longer occur in these localities. 
Such beds are often met with at the higher levels; and, in 
the lower levels, springs containing iron, which still deposit 
iron-ochre. It is very probable, that changes in the locality 
of the discharge of the springs have resulted from the ob- 
struction of their channels. Yet not only such obstructions, 
but also considerable accumulations of iron-ochre, at the 
issue of the spring itself, have, here and there, induced the 
disappearance of the springs. Thus, in one place, where 
there was a three feet thick bed of ochre, I caused a sinking 
to be made, and discovered underneath a very abundant 
ferruginous spring. 
In this neighbourhood there are, here and there, appear- 
ances from which we may conclude that there has been a 
