214 



CHAPTEE XXXIir 



CAUSES OF EARTHQUAKES AND VOLCAKOS — Continued. 



AGKNCY OF STEAM IN VOLCANIC ERUPTIONS — GEYSERS OF ICELAND EXPANSIVE 



POWER OF LIQUID GASES ACCESS OF SALT WATER, ATMOSPHERIC AIR, 



AND FRESH WATER TO THE A^OLCANIC FOCI — HOW THE SUCCESSIVE DEVELOP- 

 MENT OF VOLCANIC HEAT IN THE EARTIl's CRUST CAUSES IT TO RESEMBLE 



A BODY COOLING FROM A GENERAL STATE OF FUSION FLEXIBILITY, OF THE 



earth's CRUST ELECTRICITY AND MAGNETISM CONSIDERED AS SOURCES OF 



VOLCANIC HEAT — CHEMICAL ACTION — CAUSES OF PERMANENT ELEVATION AND 

 SUBSIDENCE OF LAND — BALANCE OF DRY LAND, HOW PRESERVED — RECAPITU- 

 LATION OF CHAPTERS XXII. AND XXIII. 



\' 



Agency of steam in- volcanic eruptions. — We have 



seen that almost all the active volcanos are on sea-coasts or 

 in islands. ' Out of 225 volcanos/ says Sir John Herschel^ 

 ^ which are known to have been in eruption within the 

 last 150 years, there is only a single instance of one more 

 thaA 320 miles from the sea, and even that one, Mount 

 Demawend in Persia, is on the edge of the Caspian, the 

 largest of all the inland seas.' JoruUo in Mexico, which was 

 in eruption in 1759, is no less than 120 miles from the nearest 

 ocean ; but, as Dr. Daubeny observes, it forms part of a train 

 of volcanos one extremity of which is near the sea. (See 

 Vol. I. p. 584, and Chap. XXVII Vol. IL p. 53.) The 

 volcano said to have been in activity in the 7th century 

 in Central Tartary is 260 geographical miles from the ocean, 

 but near a large lake. (Vol. I. p. 591.) 



Mr 



fci 



volcanos of tlie Sandwicli Islands, reminds us of the prodigious 

 volume of atmospheric water which must be absorbed into 

 the interior of such large and lofty domes, composed as they 

 are entirely of porous lava. To this source alone he refers 

 the production of the steam bv which the melted matter is 



Cn-' 



sS 



irop 



el 



that < 

 griipti' 



the ni 

 These 



the sc 



them i 



the ^ 



m 



meltec 



Tlies 



perha] 



Yoleai 



than 



somet 



on Et 



poron 



« 



ineren 



cease( 



^liicb 

 closi 



m 



^ till] 

 of 



e 



a 



posits 



fi^ 



I 



1 



