GEOLOGY. 273 



or clay-slate. Along the line of the rent, which runs east and Avest, 

 there has been a slight throw or shift of the rocks. The vein-stuff is 

 chielly Ibrined of cellular pyrites of eopj)er and iron, the porous nature 

 of which allows the hot water to percolate freely through it. it seems, 

 however, that in the continuation upwards of the same fissure little or no 

 metalliferous ore was deposited, but, in its place, quartz and other 

 impermeable substances, which obstructed the course of the 'hot spring, 

 so as to prevent its flowing out on the surface of the country. It has 

 been always a favorite theory of the miners that the high temperature 

 of this Cornish spring is due to the oxydation of the sulplmrets of 

 copper and iron, which are decomposed when air is admitted. That 

 such oxydation must have some slight effect is undeniable; but that it 

 materially influences the temperature of so large a body of water is 

 out of the question. Its effect must be almost insensible; for Prof. 

 Miller has scarcely been able to detect any sulphuric acid in the water, 

 and a minute trace only of iron and copper in solution. 



When we compare the temperature of the Bath springs, which issue 

 at a level of less than 100 feet above the sea, with the Wheel-Clifford 

 spring found at a depth of 1,350 feet from the surface, we must of 

 course make allowance for the increase of heat always experienced 

 when we descend into the interior of the earth. The difference would 

 amount to about 20Fahr., if we adopt the estimate deduced by Mr. 

 Hopkins from an accurate series of observations made in the Monk- 

 wcarmouth shaft, near Durham, and in the Dukinfield shaft, near 

 Manchester, each of them 2,000 feet in depth. In these shafts the 

 temperature was found to rise at the rate of only 1 F. for every 

 increase of depth of from 65 to 70 feet. But if the Wheel-Clifford 

 spring, instead of being arrested in its upward course, had continued 

 to rise freely through porous and loose materials so as to reach the 

 surface, it would probably not have lost anything approaching to 20 

 F., since the renewed heat derived from below would have warmed 

 the walls and contents of the lode, so as to raise their temperature 

 above that which would naturally belong t?o the rocks at corresponding 

 levels on each side of the lode. The almost entire absence of mag- 

 nesium raises an obvious objection to the hypothesis of this spring 

 deriving its waters from the sea ; or if such a source be suggested for 

 the salt and other marine products, we should be under the necessity 

 of supposing the magnesium to be left behind in combination with 

 some of the elements of the decomposed and altered rocks through 

 which the thermal waters may have passed. 



Hot springs are, for the most part, charged with alkaline and other 

 highly soluble substances, and, as a rule, are barren of the precious 

 metals, gold, silver, and copper, as well as of tin, platinum, lead, and 

 many others ; a slight trace of copper in the Bath waters being ex- 

 ceptional. Nevertheless, there is a strong presumption that there 

 exists some relationship between the action of thermal waters and the 

 filling of rents with metallic ores. The component elements of these 

 ores may, in the first instance, rise from great depths in a state of 

 sublimation or of solution in intensely heated water, and may then be 

 precipitated on the walls of a fissure as soon as the ascending vapors 

 or fluids begin to part with some of their heat. Almost everything, 

 save the alkaline metals, silica, and certain gases, may thus be left 



