Mr Poulett Scrope's Considerations on Volcanos. 335 



active volcanos of -iEtna, Vesuvius, and Stromboli, and exploring the ex- 

 tinct craters of Auvergne, of Italy, of the Rhine, and of the North of Ger- 

 many, we offer our readers some pledge for the accuracy of its facts, and 

 the soundness of its reasonings. 



The work commences with a descriptive account of the Volcanic Pheno- 

 mena, in which the author treats of the number and dispersion of volcauic 

 vents on the surface of the globe, a detailed catalogue of which is given 

 in an appendix. The number already known is supposed very much 

 within that which really exists, for many reasons, but particularly be- 

 cause the intervals of rest between eruptions are often of such long du- 

 ration that the former activity of the vent is unrecorded, and again, be- 

 cause it is probable that numerous vents exist under the sea, whose acti- 

 vity, however frequent, cannot be made known to us till the peak of the 

 volcano rises to within a short distance of the surface. Volcanic pheno- 

 mena are classed into subaerial, or those which take place in the open air, 

 and subaqueous. Of the former class, which is more open to study than 

 the latter, some take place from new, others from habitual vents. The 

 last are most common, and most accessible to observation. The condi- 

 tion of all habitual volcanos, or sources of erupted matter, appears to be- 

 long to one or other of the three following phases : 



1. In which the eruption is permanent (as in Stromboli, &c.) 



2. In which eruptions are frequent, prolonged, and of moderate vio- 

 lence, and the intervals of repose short. 



3. In which intense eruptive paroxysms, of brief duration, alternate with 

 lengthened intervals of quiescence. 



These phases are separately considered, and examples given of the phe- 

 nomena of each, as well as a particular description of all the remarkable 

 circumstances which accompany and characterize a volcanic paroxysmal 

 eruption, and which appear to the author to present so great an uniformi- 

 ty in all places, and at all times, as to warrant the conclusion that the 

 main phenomena are invariably the same ; " no farther discrepancies ex- 

 isting, than what are fairly referable to the modifications produced by lo- 

 cal accidents, or by differences in the intensity of volcanic force developed, 

 and in the mineral quality of the erupted substances." 



In Chap. II. the immediate causes of these phenomena are investigat- 

 ed; and it is observed, that all their circumstances, as well as the direct 

 observations of the author himself, Spallanzani and others, go to prove 

 the existence, between every volcanic vent, of a mass of lava, or crystal- 

 line rock in a state of actual ebullition ; the generation, or expansion, of 

 electric fluids within its interior producing intumescence and elevation, 

 and the explosions which take place from its surface. The nature of this 

 elastic fluid has been ascertained by direct experiment, and it appears to 

 consist almost wholly of aqueous vapour or steam. The uniform dissemi- 

 nation of air-vesicles through many lavns proves the vapour to have been 

 generated throughout every part of their mass. We must then suppose 

 the existence of water in combination with the other elements of the rock. 

 This leads to an examination of the nature of lavas; and the author finds 



