210 Proceedings of the Royal Physical Society. 



probably that the highest point in the surface marks a 

 position which at one time lay above the focus of the group 

 of thermal springs to which the deposits of ore are supposed 

 to be due, and around which the temperature of the thermal 

 waters was lower at the same plane above the sea -level. 

 Mineral substances deposited in the fault-fissures around, 

 would of necessity be laid down in accordance with the 

 position of the depositing temperature proper to each species ; 

 and if all the zones where deposition took place are connected 

 by imaginary lines, these would be found to outline, as it 

 were, a series of shells, concentric to the focus of the hottest 

 spring, and with those minerals lowest which require the 

 highest temperature to keep them in solution, and the others 

 at various higher horizons, whose positions are determined 

 by the factors already noticed. 



It may be remarked that these observations and specula- 

 tions, although of a very general character, are based upon a 

 long and careful examination, both at the surface and in the 

 mines, of a very large number of metalliferous deposits. 



Hot springs vary in temperature from time to time. 

 Hence it follows that, in the case of a rise of temperature, 

 previously-formed minerals are wholly or in part dissolved, 

 and their constituents are redeposited at higher levels; or, 

 with a fall of temperature, minerals whose depositing tem- 

 perature is low, are laid down upon others which, at an 

 earlier stage, were deposited from hotter water. 



All mineral veins appear to have been formed during 

 periods of terrestrial disturbance, apparently during those 

 accompanying upheaval. With any elevation of the land 

 the rock-masses are subjected to lateral tension, and hence 

 the cheek, or walls, of pre-existent faults are stretched wider 

 apart with each successive uplift — except in those cases 

 where compensation ensues through wedges of rock slipping 

 downwards and thereby filling up the vacant space. Such 

 lateral severances, if they happen to occur at a time when 

 the fissures are being coated with mineral veins, stretch the 

 previously-formed veins asunder, and permit of new deposits 

 being laid down between the dissevered walls. Hence arises 



