CHAPTEE XIII. 



THE MANIFESTATIONS OF VOLCANIC ACTION. 



Historic Records. The circumstances connected with volcanic erup- 

 tions are either mutually dependent, or naturally connected with each 

 other. For, whether exhibited as outbursts of ashes, lava-flows, mud 

 volcanoes, sulphur springs, geysers or hot springs, they are attributable 

 to the action of heat upon water, beneath the surface of the earth ; 

 and the phenomena only differ with the different conditions under 

 which the heat is developed, and the water gains access to the sub- 

 terranean regions. We obtain the clearest conception of the modes of 

 action of these agents, in their geological relations, by examining the 

 history of the origin of new volcanoes. Some of these are marine, 

 others are on land ; and although these new outbursts appear to differ 

 fundamentally from older volcanoes, in having been in eruption but 

 once, it may well be that there is nothing exceptional in such a 

 circumstance, since the most celebrated volcanoes have had long inter- 

 vals of repose. 



Graham Island. On the shallow sea-bed of the Mediterranean, 

 between Tunis and Sicily, at about sixty miles from Sciacca, and 

 thirty miles from the island of Pantellaria, an eruption took place in 

 1831 which built up a submarine cone. This volcano, when largest, 

 rose above the sea to a height of 200 feet, and attained a circumference 

 of three miles. Formed, however, of loose materials, it rapidly 

 wasted away under marine attrition, and its position is now only 

 marked by a shoal. Such an eruption is peculiarly instructive, as 

 demonstrating how unsubstantial is the fabric of which a volcanic cone 

 consists, and as indicating the nature of the evidence for the former 

 existence of a submarine volcano, which the geologist might expect 

 to find when the consolidated core left at its base by denudation was 

 enveloped by sediments. 



Jorullo. A scarcely less instructive history records the eruption 

 which formed Jorullo in Mexico in 1759. Prior to that date, the 

 farm of Jorullo was laid out in sugar-cane and indigo. It was situate 

 more than 100 miles from the coast, about 2500 feet above the sea, 

 and far away from any active volcano. This farm was bordered by 

 two streams, named Cuitimba and San Pedro. The first indication of 

 disturbance consisted in hollow subterranean sounds, accompanied by 

 constant earthquakes, lasting for about two months ; when, after a 



