852 Prof. Bischof on the Temperature of 



Besides the accidental causes which may lower the tempera- 

 ture of thermal springs, such, for instance, as meeting with cold- 

 er waters, there seems to be another very important one, viz. 

 that mineral springs frequently fill up their own course by a 

 partial precipitation of their constituents. If such a precipita- 

 tion takes place in the lowest part of the spring'*s course, and ad- 

 vances gradually higher and higher, the consequence naturally 

 must be, that the meteoric waters, no longer able to sink down 

 to so great a depth, by degrees become less and less heated. The 

 deposit from the Carlsbad springs may here be cited as an ex- 

 ample ; although in this case the deposit is only observed on the 

 surface, and the cause of its precipitation above cannot so easily 

 exist below. The enormous deposits of travertin are well known. 

 In that occasioned by the hot springs of San Filippo, fissures of 

 30 feet deep and 150 toi^OO feet long are observed. A particu- 

 larly important and direct proof is the calcareous deposit in the 

 Roman aqueduct between Cologne and Treves. It has been de- 

 posited since the time of the Romans, from several fresh water 

 springs, and is seven or eight inches thick in some places, so 

 that it has been used for columns of churches. 



I have elsewhere* called attention to the veins of brown iron- 

 stone in the trass of the Brohiihal, which may often be traced 

 to a great distance on the bare walls of ravines and quarries in 

 that rock, and which certainly owe their existence to no otl'.er 

 cause, than that ferruginous waters, of which there are still a 

 great many in existence in that district, formerly flowed in those 

 cracks, and by degrees deposited hydrate of iron, thereby filling up 

 their own course. Such fillings up with hydrate of iron are also 

 found in the grey wacke, in basalt, in trachyte, f and other rocks. 

 Inferring from the deposits of spherosiderite from mineral springs 

 actually observed, I have endeavoured to shew that veins of 

 carbonate of iron and brown ironstone may also often be the se- 

 diment of mineral springs. 



I have already proved J that when springs contain any sul- 



• Schweigger-Seidels n. .Tahb. der Chemis und Phys. vol. viii. p. 431. 



•f- Vues et coupes des principaux formations g6)logique? du Dep. du Puy- 

 ne-D6me, &c., par Lecoq et liouillel, Clermont Ferrand, 1830, 8me. livrai- 

 son, p. 223. 



$ A. O. vol. iv. p. 386. See also Lcn^champ in the Annals de Chim. et de 

 Phys. vol. xxxii. p. 294. 



