454 



THE GEOLOGIST. 



Tiiis solvent power is mauifested more especially -wlieu the water is impregnated 

 with certain gases, or Avith 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. 



Eegarding the difference of the substances held in solution by hot mineral 

 springs, and those found in metalliferous veins, M. Eiie de 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 are 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 influence 

 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, Mr. 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 gangue-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 even a brief extent of the 

 sulid contents of the vein is removed, it becomes one of the most diflicult practical 

 problems in mining engineering 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." 



He 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 surface must cause at all periods, 

 and throughout an infinite succession of geological epochs, a preponderance of 

 calcareous matter in the newer as contrasted with the older formations." 



