TRUE VOLCANOES. 483 



themselves. An investigation which has now been long 

 looked for in vain, founded on accurate experiments, ex- 

 clasively directed to these escaping gaseous fluids, would 

 lead to an invaluable extension of our knowledge of the 

 geology of volcanoes, if at the same time attention were paid 

 to the operation of the sea- water in subterranean formations, 

 and to the great quantity of carburetted hydrogen belonging 

 to the commingled organic substances. 



The facts which I have brought together at the end of this 

 section, the enumeration of those volcanoes which produce 

 pumice without obsidian, and those which yield a great deal 

 of obsidian and no pumice, the remarkable, not constant, 

 but very diversified association of obsidian and pumice with 

 certain other minerals, early led me, during my residence in 

 the Cordilleras of Quito, to the conclusion that the formation 

 of pumice is the result of a chemical process, which may be 

 verified in trachytes of very heterogeneous composition, 

 without the necessity of a previous intervention of obsidian 

 (that is to say, without its pre-existence in large masses). 

 The conditions under which such a process is performed on a 

 large scale, are perhaps founded (I would here repeat) less on 

 the diversity of the material than on the gradation of heat, 

 the pressure determined by the depth, the fluidity, and the 

 length of time occupied in solidification. The striking, though 

 rare, phenomena presented by the isolation of immense sub- 

 terraneous pumice-quarries, far from any volcanic structures 

 (conical and bell-shaped mountains), lead me at the same time 

 to conjecture 22 that a not inconsiderable perhaps even, in 

 regard to volume, the greater, number of the volcanic rocks 

 have been erupted, not from upraised volcanic structures, 

 but from a net-work of fissures on the surface of the earth 

 frequently covering over in the form of strata a space of many 

 square miles. To these probably belong those masses of 

 trap of the lower Silurian formation of the south-west of 

 England, by the chronometric determination of which my 

 worthy friend, Sir Roderic Murchison, has so greatly in- 

 creased and heightened our acquaintance with the geological 

 construction of the globe. 



22 See above, pp. 308, 330 332 336, 344 346, 354. For particulars 

 respecting the geographical distribution of pumice and obsidian in 

 the tropical zone of the New Continent, see Humboldt, Essai G&ognos- 

 tigue sur le gisement des Roches, <kc., 1823, pp. 340342, and 344347, 



2 i 2 



