Henry Woodicarcl — On Volcanos. 



351 



gas, sulphates of lime and soda, and chlorides of sodium and magne- 

 sium. The discharge of water and mineral ingredients is alike 

 uniform and constant from century to century. The temperature re- 

 mains constant at 120° Fahr. 



These Baths were well known to the Eomans, and the old Sana- 

 torium of Aqu^ Solis is stili marked by many Roman remains. 



If the solid matter brought up in solution by the Bath waters 

 could be accumulated for one year, they would form a square column 

 9 feet in diameter, and 140 feet in height. Yet all this solid matter 

 is conveyed away in a limpid stream to the Avon, and by the Avon 

 to the sea. (Lyell's " Principles," vol. i., p. 398.) 



What is the cause of thermal springs ? Water descending to deep 

 levels in the strata meets at some point with steam at a high 

 temperature, which, being converted into water by contact, raises 

 the temperature of the water, which in turn, as the store of heat is 

 accumulated, rises by rents and fissures to the surface in the form 

 of thermal springs. 



There seems no doubt that hot springs have a direct connexion 

 with volcanos. 



1. Hot springs are present in all volcanic areas. 



2. Where not connected directly with volcanos, they are found 

 situated, as in the Pyrenees, the Alps and the Himalayas, upon lines 

 of dislocation and disturbance where volcanic force, if not visible at 

 the surface, has been in operation far down beneath. 



3. Hot springs distant from volcanic disturbances are nevertheless 

 affected by them. Thus the " Source de la Peine," at the baths 

 of Luchon, in the Pyrenees, was raised suddenly during the great 

 earthquake of Lisbon in 1755, from a tepid spring to 122° Fahr., a 

 heat which it has since retained. 



Although springs, as a rule, carry carbonate of lime and sulphate 

 of lime in solution, the hotter thermal springs alone contain large 

 quantities of silica in solution. For example : the hot s]3ring of St. 

 Michael, in the Azores, having a basin 30 feet in diameter, is sur- 

 rounded by layers of travertin many feet in thickness, composed of 

 siliceous matter deposited on wood, reeds, ferns, etc. 



Fig. 10. — Section through Basin and Terraces of siliceous sinter. Hot-springs 

 Te Tarata, Rotomahana, New Zealand. (See Hochstetter's New Zealand, p. 137.) 



a. a. Level of water in repose in great Basin. 



b. b. Series of Basins into which the overflow water from the great Basin is discharged during 

 eruptions. 



c. Level of the river, d. Siliceous sinter and travertin deposited by the evaporation of the water. 



The hot springs of New Zealand (Fig. 10) are, perhaps, the finest, 

 exceeding even the Great Geyser in Iceland, which also deposits 

 enorraous quantities of silica from its waters on cooling, originally 

 held in solution. 



