On Underground Temperatures. 



31 



is general is shown by the circumstance that the upper part of all 

 these lodes consists near the surface of a crust, several feet and some- 

 times several fathoms thick, composed of the oxidised products of 

 copper and iron sulphides ; this part of the vein is known in Cornwall 

 under the distinctive term of gossan. In these cases, the water is 

 commonly impregnated with some of the resulting soluble sulphates, 

 and has its temperature raised by this decomposition. 



Mr. R. Hunt mentions* two marked instances of the heating effects 

 arising from this cause. In one case the temperature in the level of 

 a copper mine stood at 100° F., but on the removal of a very large 

 deposit of the copper pyrites " the level became cold enough to make 

 the agent wish for a great-coat." The exact difference is not given. 

 In another case, a large deposit of iron pyrites was opened at about 

 half a mile distant from a hot lode, 1530 feet deep, in the Clifford 

 Amalgamated Mine, and the mere fact of opening the mine there and 

 removing the iron pyrites, considerably reduced the temperature in 

 the mine. When it was closed the temperature rose to its former 

 height. Springs of various degrees of heat (one was as high as 124°) 

 are often met with. 



The miners of Cornwall have long held that the lodes containing 

 tin are, at equal depths, colder than those in which copper ores occur ; 

 a fact which is no doubt due to the facility with which the cupreous 

 pyrites decomposes. 



There must also be cases in which water from greater depths is 

 brought up along lines of fissure (lodes and cross-veins) ; for as 

 these are prolonged downwards, they may traverse at greater depths 

 strata, veins, or faults, charged with water, which, when thus tapped, 

 will outflow at any levels lower than the height at which the water 

 stands in the supplying source. 



It sometimes also happens that at the same depths, but in distant 

 parts of the same level, the water is of different temperatures. In 

 one instance there were springs at 102°, 110°, and 124°, and in another 

 case (Wheal Wreath), the temperature of a small stream at the east 

 end of a lode (tin) was 7l°*5, while a spring at the west end of the 

 lode had a temperature of 75° (both being at the depth of 1422 

 feet). 



On the other hand, when the water is in considerable volume, and 

 percolates rapidly, it must tend to have a lower temperature than 

 the normal rock temperature, a s in the instance where, in two adjacent 

 mines, large streams both coming out of veins, had the same tem- 

 perature of 67*5° at the respective depths of 588 and 722 feet. The 

 observations of Hen wood likewise tend to show that the range of 

 temperature of water in the same level is subject to great variation. 



In consequence of the uncertainty attaching on the one hand to 

 * Coal Commission Report, A 4 — 10. 



