30 



Prof. J. Prestwich. 



ground in the Grwennap district is of much interest. The mines in 

 that district, over an area of 5500 acres, were combined for drainage 

 purposes. A great adit carried away the water above the sea-level, 

 while a deep-seated level collected the water at the depth of from 

 1100 to 1200 feet. At the time that Mr. Hen wood wrote, the former 

 was discharging 1475 cubic feet per minute, and from the latter 909 

 cubic feet were being pumped up, or together above 10 million 

 gallons in the twenty-four hours. Taking the mean temperature of the 

 surface at 50°, as the water issues at a temperature of from 60° to 

 68° F., or at an average of more than 12° above the mean .of the 

 climate, it is easy to conceive how large must be the amount of heat 

 which the waters abstract from the mines, and how considerable the 

 cooling of the enclosing rocks which must result therefrom. 



Another observer, writing a few years earlier, states that the dis- 

 charge at the Huel Vor Mine from a depth of 950 feet, was 1,692,660 

 gallons every twenty- four hours ; at Dolcoath Mine, from about 1400 

 feet, 535,173 gallons; and at Huel Abraham Mine it reached the large 

 quantity of 2,098,320 gallons* 



Mr. Henwood remarks that "the largest streams of water flow 

 through cross-veins ; small ones through the lodes, whilst but little 

 issues from the rocks whether granitic or slaty." 



Where the water dribbles slowly through the rocks to great depths, 

 it will no doubt acquire the normal temperature of the depth, but where 

 it passes more rapidly through the veins and lodes, the temperature 

 will depend upon the time occupied in transit and on the volume of 

 water. If the flow is rapid, as it evidently is in some mines, the 

 surface-waters may carry the influence of the above-ground tempera- 

 ture to considerable depths. If on the other hand, the vein is one in 

 which the ore is subject to decomposition by the surface-waters, those 

 waters will have their temperature more or less raised. A copious 

 stream of warm water is considered among the Cornish miners a 

 favourable indication of the proximity of a lode. Nevertheless, 

 Mr. Henwood, who, as we may feel assured, fully understood all the 

 contingent conditions, considered that by a careful selection of the 

 underground springs and by taking them when freshly opened, they 

 gave safer temperature results than did the undisturbed rock. 



Hot Springs. — These are not uncommon in metallic mines. They 

 are due to two causes, lstly, to chemical decomposition ; 2ndly, to 

 water coming from greater depths. 



The first of these causes is a very general one — especially in copper 

 and iron mines, in which the lode consists of iron and copper pyrites. 

 The surface-waters decompose these sulphides, converting them into 

 sulphates, which by further changes that need not be here described, 

 pass into the oxides and carbonates of these metals. That the action 



* Dr. Forbes, " Trans. Eoy. Geol. Soc. Cornwall," vol. ii, p. 167. 



