700 DYNAMICAL GEOLOGY. 



places, arising from the action of sulphuric acid on the alumina 

 and alkali of the lavas. A decomposition of the lavas is another 

 consequence, producing gypsum (or sulphate of lime) through the 

 action of the sulphuric acid on the lime of the feldspar or pyr- 

 oxene, and also setting the silica free to make incrustations in the 

 form of opal or quartz, or siliceous earth. Carbonic acid is some- 

 times given out in such places, where there is limestone below to be 

 decomposed, — an acid (either sulphuric acid or silica in solution) 

 setting free the carbonic acid by combining with the lime. 



2. Hot springs. — Hot springs are common in volcanic regions. 

 The waters may be pure, or of a mineral character. In Tuscany 

 they give out boracic acid. In Iceland they are large and move in 

 intermittent jets, and are called Geysers. The tossing of the water, 

 which is in some cases to a height of 200 feet, is supposed to be 

 owing to a sudden production of steam in chambers beneath. The 

 stream, like any other subterranean stream, may have its head in 

 the mountains. But it comes in contact with the hot rocks, and 

 the heat and geyser-movement is the consequence. It has been 

 suggested that the waters are temporarily in the spheroidal state 

 from contact with the lavas below ; and as they increase by addi- 

 tions, after an interval, they suddenly fall below the temperature 

 requisite for this state, and then the explosion or jet takes place. 



The waters decompose the lavas, and take up the silica, owing to 

 the heat, and the presence of a little alkali derived from the feld- 

 spar of the lavas. This silica is deposited around the sides of the 

 vents, forming a neat bowl-like crater with low sides, and covering 

 a large region in the vicinity with siliceous depositions, besides 

 petrifying wood. There is a large geyser-region in New Zealand, 

 and another in California on the border of the desert. At the latter 

 boracic acid is given out, as at the Tuscan lagoons. 



When the ejection is in a muddy area, as in California, it forms 

 mud-cones. 



6. Source of volcanoes. 



The internal fluidity of the globe, or of great regions beneath the 

 outer crust, being proved, volcanoes are naturally regarded as out- 

 lets to the surface of the interior fluid. They mark the points 

 where the vaporizable materials of the interior, which naturally 

 work upward, rise through the hardened crust with the lavas they 

 inflate. Prevost uses the homely comparison of a molasses-cask in 

 which the fermenting molasses is working out at the bung-hole, — 

 noting only this difference : that the vapor which does the work 

 has another source than fermentation. The occurrence of volca- 



