OEIGnsr AND DEVELOPMENT OF ORE-DEPOSITS. 353 



Deposition of such substances as calcite in lode-fissures and 

 cavities is easy to understand, since waters charged with carbonic 

 acid will readily dissolve them out from rock masses when 

 present, and a slight lowering of temperature or pressure wiH 

 cause them to be re-deposited wherever a suitable cavity exists. 

 The solution and redeposition of quartz and of many silicates 

 can be and no doubt often is effected in a precisely similar 

 manner, and the same may be said of such substances as cuprite, 

 malachite, and chalybite, except that the solution will be 

 sometimes preceded by the oxidation of pre-existing sulphides. 

 These solutions and re-depositions will no doubt be more rapidly 

 effected in proportion as the differences of temperature and 

 pressure are greater, but ordinary temperatures and pressures 

 will suffice, given sufficient time. 



The relative depths to which the waters will penetrate before 

 finding their way into fissures, and the particular parts of the 

 fissures in which deposit takes place, will determine whether 

 such deposits should be regarded as the results of infiltration 

 from above or of lateral secretion, but it is quite evident that 

 the transference of substances of groups 1 to 3 may take place 

 without the solutions making their way to any great depths, and 

 consequently -without any notable elevation of temperature or 

 considerable pressure. 



More than 20 years ago Mr. Robert Hunt wrote as follows with 

 regard to infiltration from above. " The view supposes waters to 

 have penetrated from above, and that, in passing through the 

 rock-fissures which were the natural channels of aqueous circu- 

 lation these waters deposited, under the influence of what Sir 

 Henry De la Beche called 'rock conditions,' and which Mr. Robert 

 Were Fox and M. Becquerel referred to electrical action, their 



metallic and earthy salts forming the lodes as we find them 



my leaning is towards this hypothesis."* In expressing this 

 opinion, Mr. Hunt does not specially indicate the sources of the 

 "metallic and earthy salts" which are conveyed by the circulating 

 waters, but as he speaks of water " penetrating from above," we 

 may fairly assume that they would be free from metallic salts, at 

 least at the start, and that they would acquire their solid contents 



* Trans. Boy. Oeol. 8oc. Corn., ix, p 22. 



