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in all probability it originated during the last upheaval of the 

 district, when the volcanic forces that had produced so important 

 effects in Miocene times were gradually expending themselves in 

 the formation of hot and mineral springs. These forced their way 

 upwards along the old lines of weakness and deposited the minerals 

 held in solution as soon as the temperature of the upflowing current 

 was reduced to the depositing point by contact with colder 

 currents flowing downwards from the surface or by actual proximity 

 to the surface itself I have also been led to conclude that the 

 lead ore, at the close of the period of its formation, extended 

 through the rocks of our district as shallow, detached vein deposits, 

 whose general contour, if the disconnected deposits could be joined 

 over large areas, would be that of irregularly concavo-convex sheets 

 or inverted saucer-shaped masses. The way the ore works down- 

 wards as we recede from particular centres seems to point to these 

 centres marking approximately the position of the principal seats 

 of hydrothermal action ; as, where the temperature of the thermal 

 springs rose highest it would be there that the mineral matters 

 would rise to the parts nearest to the surface before the temperature 

 of the medium was reduced to the depositing point. Of course, 

 where the form of the surface, or the lie of the rocks, favoured the 

 accumulation of larger quantities than usual of water derived from 

 the surface, this cause would act in an opposite direction, and 

 induce deposition at a lower horizon than would otherwise be the 

 case. Variations in the thermal conductivity of the adjoining 

 rocks would also produce important modifications of the same 

 kind of effect. It is causes of this nature that have led, I believe, 

 to the ore in one part of a district occurring only in rocks high up 

 near the tops of the fells, and in another in a part of the same 

 series lower by many hundreds of feet ; and, again, to the ore 

 occurring exclusively in connection with limestones in one area, 

 while elsewhere it is found only in arenaceous beds. The same 

 causes will help to explain why the concurrence of a downfold of 

 the rocks with a dislocation should have been favourable to 

 mineral deposition, and in another, why it seems to have exercised 

 no perceptible influence at all. The whole body of facts seem to 



