THERMAL SPRINGS. 193 



region, are not always indebted for their entire freedom 

 I'rom snow to the steepness of their sides, but to other 

 causes, I buried the bulb of a thermometer only three inches 

 deep in the sand which filled the fissure in a ridge on the 

 Chimborazo at an elevation of 18,290 feet, and therefore 

 3570 feet above the summit of Mont Blanc. The thermo- 

 meter permanently showed 10.5 F. above the freezing point, 

 whilst the air was only 4.5 F. above that point. The re- 

 sult of this observation is of some importance ; for even 

 2558 feet lower, at the lower limit of perpetual snow of the 

 volcano of Quito, according to numerous observations col- 

 lected by Boussingault and myself, the average temperature 

 of the atmosphere is not higher than 34. 9 F. The ground 

 temperature of 42^.5 must therefore be ascribed to the sub- 

 terranean heat of the doleritic mountain : I do not say of 

 the entire mass, but to the currents of air ascending in it 

 from the depths. At the foot of Chimborazo, at an eleva- 

 tion of 9486 feet towards the hamlet of Calpi, there is, more- 

 over, a small crater of eruption, Yana-Urcu, which, as indeed 

 is shown by its black, slag-like rock (augitic -porphyry), ap- 

 pears to have been active in the middle of the fifteenth cen- 

 tury. 48 



The aridity of the plain from which Chimborazo rises, 

 and the subterranean brook, which is heard rushing under 

 the volcanic hill (Yana-Urcu) just mentioned, have led 

 Boussingault and myself 43 at very different times to the idea 

 that the water which the enormous masses of snow produce 

 daily by melting at their lower limit, sinks into the depths 

 through the fissures and chambers of the elevated volcano. 

 These waters perpetually produce a refrigeration in the 

 .strata through which they run down. Without them the 

 whole of the doleritic and trachytic mountains would ac- 

 quire, even at times when no near eruption is foretold, a still 

 higher temperature in their interior, from the volcanic 

 source, perpetually in action, although perhaps not lying at 

 the same depth in all latitudes. Thus, in the varying 

 struggle of the causes of heat and cold, we have to assume 

 a constant tide of heat upwards and downwards in those 

 places where conical solid parts ascend into the atmosphere. 



4J Humboldt, Kleinere Schriften, Ed. i, pp. 139 and 147. 



'' Humboldt, Op. tit., s. 140 and 203. 



VOL. V. O 



