454 
THE GEOLOGIST. 
This solvent power is manifested more especially wkcn the water is impregnated 
with certain gases, or witb. alkaline solutions, the oxides of metals being most 
freely acted upon by the latter ; and, as all the primitive and many other rocks 
contain potash, it is easy to perceive that there is abundant provision in nature 
for the solution and transportation of metals through the medium of aqueous 
currents, in the veins. 
The original spaces of decomposed crystals of the feldspar, forming an in- 
gredient of the natural rock, are not unfrequently refilled with peroxide of tin ; 
even part of the original feldspar sometimes remaining, according to the extent to 
which the disintegration has proceeded ; and sulphate of baryta, a substance 
usually termed insoluble, and common in lead-mines, occurs under circum- 
stances which leave no doubt of its having been deposited from aqueous 
solutions. It has been known to replace portions of corals in the mountain- 
limestone of Derbyshire. 
Regarding the difference of the substances held in solution by hot mineral 
springs, and those found in metalliferous veins, M. Elie do Beaumont has pointed 
out the reason why, notwithstanding the intimate relations of such springs with 
the formation of mineral veins, the contents should not be expected to be 
identical : the fact being that those substances which arc least soluble are not 
discharged by the hot springs, because in the flow upwards of the thermal water, 
each change of temperature in cooling, as they mount towards the surface, causes 
a precipitation of the least soluble substances on the walls of the fissure, while 
the alkaline salts and other readily soluble substances are retained in the water, 
and discharged by the springs. 
The second point considered is the mechanical action of water. This inQucnce 
in the degradation of rocks and transportation of particles of matter is so commonly 
apparent that we need not dwell on it in this review. Two points noticed are, how- 
ever, worthy of insertion. Referring to various conditions under which silica is 
deposited. Mi'. Leeds continues, that the "vein-fissures may owe their origin 
to the gradual deposition of the gangue-stone forcing asunder the wall-rock. 
The expansion of a small quantity of water in a crevice while passing by freezing 
to a solid state will dissever large masses of rock. The gradual deposition of the 
silica into the crevice may, by passing from a fluid to a solid state, widen the 
fissure and enlarge the vein. Such a course of production would account for the 
presence of those fragments of wall-rocks which are found in the body of the 
vein ; they have been left in their position, enclosed in the gaugue-stone, by the 
gradual receding of the wall-rock. Had the vein been an open fissure from the 
first, those fragments must have fallen to the lowest depths ; nor would the wall- 
rocks have been able to sustain their position — the ' hanging-wall ' or roof of the 
vein must have fallen to the ' foot-wall ' or floor. When eveu a brief extent of the 
solid contents of the vein is removed, it becomes one of the most dithcult practical 
problems in mining enginceriug to sustain the walls in their original position. 
How utterly impossible it would have been for them to sustain that position, if 
without support throughout their whole extent." 
lie then quotes from our countryman, Sir Charles Lyell, in reference to 
calcareous springs, that " the constant transfer of carbonate of lime from the 
lower or older portion of the earth's crust to the sui-face must cause at all periods, 
and throughout an infinite succession of geological epochs, a preponderance of 
dleareous matter in the newer as contrasted with the older formations." 
