oS ate 
Natural History of Volcanos and Earthquakes. 253 
stances are exhibited, which indicate that these rocks are of very 
different ages.* 
If the activity of a volcano ceases, but the channels by which 
the waters enter remain open, the volcanic action may be replaced 
by hot springs.+ In this case it is easy to conceive that the me- 
teoric waters, continually sinking into the hot interior, would 
there assume the surrounding high temperature, and rise again to 
the surface with a temperature, diminished proportionally to the 
decrease of pressure, either through the former lava-channels, or 
other fissures more recently opened.{ But if at that depth, the 
hydrostatic pressure be greater than the elastic force, which the 
Water has there acquired, no steam will be generated in the whole 
Course of the spring; but, in the contrary case, from the lowest 
Point up to the point where the elastic force becomes greater than 
the hydrostatic pressure, the water will escape in the form of a 
Vapor. However high the temperature of the water may be at 
the lowest point of its course, whether in the liquid or in the 
gaseous state, yet, when it reaches the surface, it cannot exceed 
the boiling point. The reason of springs but seldom attaining 
even this maximum, may be either the loss of heat communicated 
to the superior strata of the earth, or that they meet with streams 
of gas, (carbonic acid, or sulphuretted hydrogen,) which, even if 
ed of very high temperature, will cause a depression of 
their temperature, as is proved by experiments cited in Chap. IT, 
of Memoir on Springs, p. 336, vol. xx, Ed. Phil. Journ.¢ The 
Production of hot springs, according to the last species of volcanie 
aa ee 
‘ning from east to west, parallel to the general line, of voleanos in Mexico. 
—$ Von Humboldt is also of opinion, Reise, &c. t. i, p. 187 and 188, that the va- 
Por which rises from the “‘ Narices del Pico” as they are called, and from the rents 
the crater of Teneriffe, is nothing but atmospherical water which has penetrated 
by infiltration. 
§ According to M. Arago, the hottest spring in Europe unconnected with mod- 
Volcanic action is that of Chaudesaigues in Auvergne, whose temperature he 
{Motes at 176° Fah. Annuaire du Bureau des Longitudes, 1836. The next hot- 
3 to this seems to be Thuez, in the Pyrenees, whose temperature is, according to 
Tofessor Forbes, 171°.5 Fah. Phil. Trans. t. ii, p. 603, for 1826. Forbes believes, 
a 610, the baths of Nero, near Naples, the hottest spring on the continent of Eu- 
eee is connected with modern volcanic action, the temperature being 
oF. 
