MODE OP FORMATION OF ORES. 565 







Silica, when brought in by waters containing alkaline carbonates, in 

 which it is notably soluble, might form silicates of the alkalies, the carbonic 

 acid of the latter serving, as suggested in the case of sulphides, to render 

 the waters capable of carrying away the earthy carbonates. Later the com- 

 bined alkalies might be in part replaced by other bases, such as oxide of 

 iron, and in part actually dissolved out, leaving free silica. 



MODE OF FORMATION. 



It has been assumed that the ores of this region were originally 

 deposited, first, from aqueous solutions; secondly, by a metasomatic inter- 

 change 1 with the country rock; and thirdly, in the form of sulphides. Direct 

 evidences of processes which went on in former geological epochs at great 

 depths below the surface are necessarily difficult to obtain, especially where, 

 as in the present case, the field of observation was confined to material 

 which has been more or less altered since those processes had ceased. It 

 is therefore necessary in the commencement to assume the more probable 

 among possible processes, and then to see to what extent the assumed pro- 

 cess may be reconciled with observed facts. 



The agencies by which mineral matter may be carried from one place 

 to another within the earth's crust are heat and water, or a combination of 

 the two. It was only in the very infancy of geology that heat alone was 

 seriously admitted to be a possible agent for the formation of mineral 

 deposits in depth. The nature of such deposits was soon found to be such 

 as to preclude the possibility that they might have resulted from the con- 

 solidation of a fused mass. Sublimation, on the other hand, as a means of 

 forming such mineral masses, involves a combination of heat, pressure, and 

 water, and may therefore in one sense be considered to be a form of aqueous 

 solution. Its practical demonstration, however, is confined to laboratory 

 experiments, which can at best be but an imperfect imitation of the process 

 of nature. The removal of the materials of which ore deposits are formed by 

 the agency of water alone may be observed to be going on in nature at the 

 present day. Hence this agency, to which, under the comparatively un- 



'By metasomatic interebange is meaiit an interchange of substance, without necessarily involv- 

 ing, as does pseudomorphism, the preservation of the original form of the substance replaced or even 

 of its original volume. 



