OEIGIN AND DEVELOPMENT OF OKE-DEtOSITS. 337 



The essential difficulty in all such cases is the same, viz. : 

 that small, and even insignificant bodies, if very near, act as 

 energetically on the dipping needle or the galvanometer as large 

 and valuable bodies at a greater distance, and also that, owing 

 to the action being inversely as the square of the distance, it is 

 too feeble in most cases to make itself evident at all, even at 

 very moderate distances. 



Mr. Fox's experiments on the influence of electricity on 

 sulphide ores as bearing on the origin of gozzans will be referred 

 to in the next section. 



Sec. 10. — Some specific effects of the underground circulation. 



The physical forces referred to in See. 8, acting by means 

 of the circulating waters (Sec. 7) produce certain remarkable 

 and wide-spread changes in the country rocks, as well as on 

 already existing metalliferous deposits. Some of the most 

 important of these will be here dealt with. 



Hydration. This is seen in the occasional conversion of 

 masses of hematite into limonite, and of anhydrite into gypsum, 

 the original structure and texture being often perfectly preserved. 

 Micaceous rocks are often found with the mica more or less 

 changed by hydration : in this way Damourite, Margarodite, and 

 some other minerals appear to have been formed. Pure water is 

 able to effect many such changes, but water charged with 

 carbonic acid, as is the case with all natural waters falling 

 through the atmosphere, acts much more thoroughly and 

 rapidly.* 



The hydrated micaceous mineral Sericite seems to have been 

 formed by circulating waters ; not however by the hydration of 

 pre-existing micas, but as a decomposition and re-composition 

 product of other non-micaceous silicates. 



Formation of Go%%an. This is essentially a process of oxida- 

 tion, accompanied in most instances by hydration. In the West 

 of England, gozzan consists mainly of hydrated peroxide of iron, 



* A. Johnstone in experimenting on Muscovite found that pui'e water had the 

 same effect as water charged with carbonic acid, viz., the mica was simply 

 hydrated ; hut with biotite, iron and some other components were carried off when 

 carbonated water was used, while it was simply hydrated by pure water. All, 

 however, increased in balk. Q. J. G. /Soc.,No. 173, p. 363. 



