174 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



other mines in Cornwall as well as in some of the Devonshire 

 mines, but have not been analysed ; the last four in Table 1 

 seem to be merely surface waters — somewhat raised in temper- 

 ature and otherwise modified slightly in passing through the 

 veins or through old and abandoned workings — so as to partake 

 in some degree of the characters of true thermal waters. All 

 were taken from points above the (artificially lowered by 

 pumping) drainage level of the country, but below the natural 

 drainage level. 



The phenomena of ore-deposits throughout the world are 

 only known to us from the surface downwards to at most 5,500 

 feet, since our deepest shafts and borelioles have not yet 

 exceeded that depth ; in fact we have very few workings at 

 present more than 2,500 to 3,000 feet from the surface. But in 

 our deepest workings streams issue whose temperature (and 

 other indications also to which reference will be made hereafter) 

 lead to the conclusion that they arise from greater depths than 

 4,000 feet. At that depth it does not seem likely — from the 

 most recent observations that the normal temperature of the 

 rocks — apart, that is from strictly local causes, would be much 

 greter than 100° F., yet in some regions temperatures of 200° or 

 more are known. The high temperature of the Wheal Clifford 

 spring does indeed seem to be due to some local cause of heating 

 which is not very deep seated — the far more gradual increase at 

 Dolcoath would seem to come much nearer to normal conditions. 

 In any case we may safely say that in the West of England we 

 have no present information of subterranean temperature by 

 means of springs which have come from a greater depth than 

 5,000 feet. 



This universal subterranean circulation, whether canalicular 

 or interstitial, cannot fail to produce the most extensive changes 

 in the rocks, and especially in a region of ancient fissures such 

 as we have in the West of England. It is therefore of great 

 importance to the geologist and still more so to the miner. 



Briefly stated, its work will be : — 

 1.— -To dissolve what can be dissolved. 



2. — To transfer material from one place to another, so facili- 

 tating mutual reactions and leading to the formation of 

 new deposits. 



