VOLCANOES. 737 



sition of the lavas thus occasioned often results in producing gypsum 

 (or sulphate of lime), through the action of the sulphuric acid on the 

 lime of the feldspar or pyroxene ; also opal, or quartz, or siliceous 

 earth, from the silica set free ; and impure clay. Carbonic acid is 

 sometimes given out in such places, when there is limestone below 

 to be decomposed, — some acid (either sulphuric acid or silica in solu- 

 tion) setting free the carbonic acid, by combining with the lime. The 

 " solfatara," miles north of Naples, is the best known and earliest de- 

 scribed of such regions. 



Fumaroles are cavities in the lavas of a volcano, or in a solfatara, 

 whence steam issues freely. The steam and acid vapors often produce 

 beds of whitish and red earth through the decomposition of the lavas ; 

 incrustations of sulphur about the vents, or beneath an outer firm layer 

 of the earth ; druses of crystals of hematite or magnetite in the cav- 

 ernous lavas, as at Vesuvius, Sicily, Stromboli, derived, it is supposed, 

 from the reaction of steam on vapors of iron chloride ; sometimes, at 

 Vesuvius, incrustations of common salt and of sal-ammoniac and va- 

 rious other products. 



3. GENERAL CHARACTER OF VOLCANIC ACTION. 



The preceding facts lead to the following conclusions : — 

 (1.) Quiet Eruptions. — In large volcanoes, like those of Hawaii, 

 when the lavas are free-flowing, eruptions may take place without earth- 

 quakes, or violence of any kind ; and be the result of gradually accu- 

 mulating pressure (1) from the increasing height of the column of 

 lavas, and (2) from the increasing amount of escaping vapors. The 

 pressure per square inch of 100 feet of the liquid lava (the specific 

 gravity of the rock being 2*90-2 *9o, but making a deduction of one- 

 tenth for liquidity and the contained vapors) is about 120 pounds ; or 

 for 3,000 feet, 3,600 pounds ; and for the height of the mountain, 

 13,760 feet, about 16,500 pounds. The great lava fountain of the 

 summit-eruption of 1852 had a head of lavas above it over 3,000 feet 

 high ; and hydrostatic pressure, as Mr. Coan suggested, may in this 

 case have been the chief force at work. 



But in general the other agency — that of evolved vapors — has 

 acted along with the pressure of the lavas. 



The great efficiency of this cause, even in Kilauea, is apparent from an occurrence at 

 the eruption of that crater in 1840. A fissure was opened to the top of the walls of the 

 pit, 500 feet above the boiling lavas of the interior, and liquid rock flowed from it down 

 the slopes to the black ledge. The vapors must have been the chief cause of such a 

 fracture, and of outflow at a height so far above the level of the crater's boiling lakes. 

 In the case of the fountains of lava near Kahuku, mentioned above, it may be that the 

 jets were produced by the access of waters to the liquid lavas beneath ; but they were 

 more probably due to the column of lavas in Mount Loa. 

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