ON ITS EXTERIOR. HOT AND COLD SPRINGS. 209 



Comangiilas," in the Mexican territory near Guanaxuato. The 

 first of these had a temperature of 90.3 Cent. (194.5 Fahr.), 

 and issued in granite; the latter in basalt, with a tem- 

 perature of 96.4 Cent. (205.5 Fahr.). According to our 

 present knowledge of the increase of heat at increasing 

 depths, the strata, by contact with which these temperatures 

 were acquired, are probably situated at a depth of about 

 7800 English feet, or above two geographical miles. If the 

 internal terrestrial heat be the general cause of thermal springs 

 as well as active volcanoes, the rocks which the waters 

 traverse can influence the temperature only by their different 

 capacity for heat and their conducting powers. The hottest 

 permanent springs (between 95 and 97 Cent., or 203 

 and 209 Fahr.) are also the purest, containing the smallest 

 portion of mineral substances in solution, but their tempera- 

 ture appears to be less constant than that of springs between 

 50 and 74 Cent. (122 and 165 Fahr.), which, in Europe at 

 least, have been found remarkably uniform, both in tem- 

 perature and mineral contents ; having undergone no change 

 for the last fifty or sixty years, or since 4 the application 

 of exact thermometric measurement and accurate chemical 

 analysis. The thermal springs of las Trincheras, on the 

 other hand, have increased about 7 Cent, (or 12 Fahr.) in 

 twenty-three years ; their temperature having been observed 

 by myself, in 1800, to be 90.3Cent. (1945. Fahr.), while 

 in 1823, according to Boussirigault,( 205 ) it reached 97 Cent. 

 (or 206.6 Fahr.) . This gently flowing source is therefore, at 

 the present time, almost 7 Cent. (12. 6 Fahr.) hotter than 

 the intermitting fountains of the Geyser and the Strokr, of 

 which the temperatures have been recently determined with 

 great care by Krug of Nidda, The elevation of the new 

 volcano of Jorullo, unknown before my American jouniev, 



