160 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



This last stage is illustrated by the phenomena of Minear 

 Downs and Cligga, already described. 



It will be seen hereafter that Mr. Fox's theory of the 

 electrical origin of many ore-deposits has a direct bearing upon 

 this question of sheeting. 



In most cases, whether the sheeted structure be observable 

 or not, there are other notable alteration phenomena of more 

 distinct chemical origin — such as discoloration by oxidation or 

 hydration ; or bleaching by lixiviation ; or softening by incipient 

 decomposition ; or hardening by infiltration of siliceous or 

 schorlaceous matter; orgeneral"mineralization"by the infiltration 

 of pyrites or other metalliferous substance; so that in some way 

 or other the approach to a good mineral deposit is always 

 heralded by signs unmistakeable to eyes M'hich are familiar with 

 the mineral phenomena of that particular district. Unfortun- 

 ately the indications vary to some extent in different districts, 

 and often they extend to but very small distances from the 

 ore-deposits sought ; frequently they are disguised by more 

 general surface decomposition, or buried beneath more recent 

 and barren strata, or under surface accumulations of detritus or 

 of vegetable soil ; so that it needs in most cases not only great 

 knowledge and experience, but also a large expenditure of time 

 and money in prospecting and in exploration work before the 

 valuable existent deposits are hit upon, unless some lucky 

 accident comes to the aid of the miner. 



The chemical changes in granites associated with metalli- 

 ferous deposits are so very marked, that Capt. Chas. Thomas 

 was in the habit of speaking of the unchanged granite (quarry 

 granite as it is often called) as primary granite, while the changed 

 rock looked upon as favourable by the miner he spoke of as 

 seeondary granite. Again our corresponding member, Mr. E. 

 Pearce, writing on the metalliferous granites* of Carn Marth, 

 remarked that the whole rock is commonly discoloured with 

 peroxide of iron ; and the felspar kaolinized, or else replaced by 

 chlorite, fluor, schorl, or oxide of tin; while a tabular structure is 

 often set up in the neighbourhood of ore-veins. In 1863 some 



*Eeport Miners Association, 1862. 



