On the Agency of Water in Volcanic Eruptions. 141 



form independent and isolated water-reservoirs ; but, traversed as they 

 are by dykes communicating with the central mass, these dykes serve 

 as so many conduits to carry the water from the separate water- 

 reservoirs and drain them into the central duct (fig. 2) whenever 

 the normal hydrogeological conditions are disturbed. At such times 

 the dykes therefore contribute greatly to the discharge of water into 

 the interior of the volcano. 



Very little is known of the substrata of a volcanic mountain. We 

 know that Vesuvius, Etna, and Hecla stand on Tertiary strata — that 

 some volcanoes in America stand on Cretaceous or Jurassic strata, 

 and others probably on crystalline rocks, but of the stratigraphical 

 details underground we have very scanty information. The only 

 instances that I am acquainted with are the sections obtained in boring 

 for the two artesian wells constructed in 1865-66 at Naples by MM. 

 Degousee and Laurent, of Paris. These supply very important data not 

 only respecting the volcanic beds, but also respecting the sedimentary 

 strata beneath. One well is situated in the Piazza Villa-Reale, and 

 was carried to a depth of 1106 feet, and the other, in the gardens of the 

 Royal Palace at Naples, 72 feet above the sea-level, was carried to the 

 depth of 1524 feet. The details given of this latter by M. Laurent 

 are as follows : — * 



Section of Artesian Well in the Palace Gardens, Naples. 



Volcanic 

 ejecta- 

 menta. 



Sub- 



Appenine <J 

 strata. 



Soil and made ground .... 



Yellow volcanic tuff 



Green ,, „ 



Volcanic ashes, in places 

 argillaceous, and containing 

 numerous pebbles of tra- 

 chyte 1st spring 



Greenish volcanic tuff .... 



6. Grey clay 



7. Grey marly tuff with tra- 

 chyte 



5. 



with 



of 



Thickness, 

 metres. 



16-50 

 52-50 

 33-00 



103-40 



7- 00 



8- 10 



4-00 



8. Sandy marl 

 lignite 



9. Grey marly and bituminous 

 sands, with mica. 2nd spring 



10. Hard sandstone 



11. Compact shelly marl . . 44*80 



12. Alternating micaceous sands, 

 soft sandstones and carbo- 

 naceous marl .... 3rd spring 



25-00 



27-00 

 1-80 



48-70 



Depth, 

 metres. 



16-50 

 69-00 

 102-00 



205-40 

 212-40 

 220-50 



224-50 



249-50 



276-50 

 278-40 

 323-20 



37190 



* " Guide du Sondeur," 2nd Edit. 1861 j vol. i, p. 137 ; ii, p. 496 ; and PI. L. 



