ON ITS EXTEHIOH. MUD VOLCANOES. 211 



The remark made by Arago, in 1821, that the deepest 

 Artesian wells are the warmest, threw new and important 

 light on the origin of thermal springs, and on the investiga- 

 tion of the law of increase of terrestrial heat at increas- 

 ing depths. ( 208 ) It is a striking circumstance, which has 

 been only recently noticed, that, at the end of the third cen- 

 tury, Saint Patricks, ( 209 ) who was probably bishop of Pertusa, 

 was led, by a consideration of the hot springs which issue 

 from the ground near Carthage, to form very correct views 

 regarding these phsenomena. To inquiries as to what might 

 be the cause of boiling water thus issuing from the earth, 

 he replied, " Tire is nourished in the interior of the earth 

 as well as in the clouds, as you may learn both from Mount 

 Etna and another mountain near Naples. Waters rise from 

 beneath the ground as in siphons ; those at a distance from 

 the subterranean fire are colder, but those which have their 

 source near the fire are heated by it, and bring with them to 

 the surface which we inhabit an insupportable degree of heat/' 



As earthquakes are often accompanied by emissions of 

 water and elastic fluids, we may recognise in the Salses, or 

 small " mud volcanoes/' a transitional phsenomenon between 

 issues of gaseous fluids and of thermal springs, and the grand 

 and awful phsenomenon of streams of lava issuing from 

 burning mountains. On the one hand, we may consider 

 the mountains as springs or fountains sending forth, in- 

 stead of water, molten earths forming volcanic rocks ; 

 and, on the other hand, we should remember that thermal 

 springs, impregnated with carbonic acid and sulphurous 

 gases, are continually depositing successive horizontal beds 

 of travertin, or forming conical hills as in Algeria in Nor 

 them Africa, and in the Banos of Caxamarca on the 



