kRPORT ON MINERAL AND THEUMAL WATERS. Gx^t 



to have commpnced ; but that when situated near to its axis, 

 they generally occur in some deep valley, and consequently at a 

 comparatively low level. 



This is the case with Barege and Cauterets in the Pyrenees, 

 and with St. Gervais in the Alps, which latter, as M. Delarive* 

 had many years ago observed, is situated exactly on the spot 

 which, of all others, unites most completely the conditions, of 

 approaching in the nearest degree to the centre of the chain, and 

 being at the same time least elevated above the level of the ocean. 



But Professor Forbes, in an interesting memoir to which I 

 have already had occasion to referf, points out other circum- 

 stances of physical constitution, which seem to characterize the 

 greater part, at least, of springs of this description. 



He has shown, by an extensive induction of particulars, that 

 the thermal springs of the Pyrenees, for the most part gush out 

 from the vicinity of intrusive rocks, such as granite, serpentine, 

 greenstone, and the like ; moreover, that the structure and po- 

 sition of the stratum through which the latter have lieen thrust, 

 are both of such a nature as to afford indications of violence. 



Several of these thermal waters he has even traced, rising ex- 

 actly from the line of junction between the granite and the 

 stratified rock. 



And this brings me to the consideration of the third circum- 3rdly. Con- 

 stance alluded to as characterizing thermal waters : 1 mean their ''guous to 



, . cxtcnsivG 



connexion with faults or dislocations. faults or 



This mutual relation is illustrated by the case of the Carlsbad dislocations. 

 springs, according to the description of them given by Von HoffJ. 



They are described by him as issuing from the bottom of a 

 narrow glen, bearing in itself the evidences of some great natural 

 convulsion. 



It lies nearly at right angles to the valleys of denudation that 

 exist in the immediate neighbourhood ; it is more narrow and 

 more precipitous than the latter ; and, as Von Hoff states, the 

 granite which forms the fundamental rock, is overlaid by a 

 breccia, made up of fragments of this rock cemented together by 

 a siliceous paste, which is in great measure covered over by the 

 calc. sinter deposited at present by the springs, but in one side of 

 the valley protrudes itself, and appears above it. 



This breccia Von Hoff attributes to the spring, which in former 

 times, like those of Iceland, may have deposited siliceous matter ; 

 but as, on a recent visit to Carlsbad, I could perceive no kind of 

 breccia that bore the appearance of having been cemented by 

 the materials of a thermal water, I am disposed to doubt this 



* Bibliotheque Britannique. f Phil. Trans, 1836. 



X Geognostiche bemerknnyen i'cber Karhhad. Gotha, 1825. 

 VOL. v.— 1836. F 



