188 C. W. MARSH. — GEOLOGICAL NOTES 



from the surface downwards. These fissure lines offering the 

 more easy passage for such percolating waters, it is obvious that 

 the more soluble mineral matters contained therein would be 

 constantly removed to lower levels and deposited only where con- 

 ditions were favourable for their precipitation, being thus as it 

 were, removed and protected from surface disintegration. The 

 solubility of metallic combinations in the presence of sulphated 

 waters, containing soda or lime, more especially if copper be pre- 

 sent, are well known and taken advantage of on a large scale in 

 metallurgy, to separate the various metals from their earthy matrix. 

 In all such experiments the more nearly we imitate natural pro- 

 cesses and conditions the greater is our success, thus clearly 

 demonstrating the most natural solutions to be the most universal 

 solvents. Such solutions have not only the power of separating 

 from their various combinations the greater number of the metals 

 found in nature, but of precipitating the same under slightly 

 changed conditions. Taking those veins for instance, the constant 

 -association of silver, lead, copper, zinc, &c, with iron, points to 

 one probable mode of their separation by deoxidation, due to the 

 strong affinity for oxygen of the ferrous salts, during their per- 

 oxidation out of contact with the atmosphere. The same results 

 may be obtained by the deoxidizing powers of organic matters in 

 the presence of such sulphated solutions, containing metallic salts 

 giving rise to sulphurets of the metals. Such a process, continuing 

 during the slow removal of a vast thickness of rock material, has 

 no doubt accumulated by downward concentration a considerable 

 portion of the metals, not only from the upper removed parts of 

 the veins themselves, but probably also from the rocks in their 

 immediate vicinity. Thirdly there are good reasons for assuming 

 the rocks in which these veins occur to be metalliferous, apart 

 from the fact that a large quantity of iron must have been set free 

 during the decomposition of the mica (apparently biotite) of the 

 surrounding gneiss and probably concentrated in the lode. The 

 presence of small quantities of silver, lead, copper, and sometimes 

 zinc, in these rocks at considerable distances from the veins, 

 points to a probable source of some of these metals. We must 

 not forget in this connection, that the chemical actions resulting 

 in the alteration of these rocks, may have taken place in the pre- 

 sence of metallic solutions derived from sources far removed, and 

 that these rocks may have become impregnated with metallic salts 

 by a natural exchange of bases, nevertheless the balance of evidence 

 points to their metalliferous character previous to their decompo- 

 sition. 



That the present horizon has, for untold ages, been removed 

 from the more active agencies of disintegration is evidenced by 

 the comparative preservation of the surface, and the extent to 



