874 Prof. Bischof on the Natural History of 



depth, and therefore are originally warm, become cooled by 

 mixture with cooler springs. 



The warmest of the mineral springs in the environs of the 

 Laacher See exceed the mean temperature of the ground by 

 *7° to 10° Fahrenheit. What is worthy of remark is, that they 

 rise from the deepest spots of the valley, where, therefore, their 

 subterraneous channels are proportionably deepest under the 

 rock, and possess already a relatively higher temperature. On 

 pursuing the mineral springs up the valley, we find that their 

 temperature decreases in a somewhat regular^'ratio.* 



The proportionably small number of clefts in the clay- 

 slate rocks may certainly account for the circumstance, that, in 

 the Laacher See, the EifeJ, and the Taunus, so few springs of 

 considerable high temperature occur, though the channels of 

 the carbonic acid gas lead down to such great depths, probably 

 to points where a red heat exists. Such warm springs may per- 

 haps owe their existence to the favourable circumstance of a 

 cleavage surface, which intersects the strata at an obtuse angle, 

 leading up from the cleft between the volcanic cone and the 

 clay-slate rock, and opening at a valley, as c d. Perhaps the 

 warm springs at Bertrich and Ems, which rise in deeply hol- 

 lowed valleys in clay-slate rocks, are thus produced. 



We may also easily conceive the possibility of obtaining a 

 thermal spring by boring. A slight glance at the figure will 

 shew that a hole bored into a clay-slate rock in a valley, in the 

 vicinity of a volcanic cone, will probably give exit to a thermal 

 spring, if the borer reach the surface of a stratum or a slate 

 surface communicating with the cleft between the volcanic and 

 the clay-slate rock. A successful attempt of this kind was 

 actually made a few years ago, by boring into the clay-slate 

 rock at the foot of the basaltic hill, the Landshrone in the Ahr 

 valley, about three German miles north of the Laacher See, 

 when a copious mineral was obtained of the temperature of 

 58° F., affording considerable disengagement of carbonic acid 

 gas. Indications prognosticating a favourable result of this 

 undertaking were indeed present, inasmuch as a mineral spring 



* So in the chain of Taw wms mountains, the warm springs rise deep in the 

 valley, the cold acidulous springs on the heights. 



