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Co. XXV.] 



HERCULANEUM AND POMPEII 



641 



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during 



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 ced in 

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a volcanic cone, alternations will be found of lava, alluvium, 

 and showers of ashes. The great eruption, in 1822, caused 

 a covering only a few inches thick on Pompeii. Several feet 

 are mentioned by Prof. J. D. Forbes/ but he must have 

 measured in spots where it had drifted. The dust and ashes 

 were five feet thick at the top of the crater, and decreased 

 Gradually to ten inches at Torre del Annunziata. The size 

 and weight of the ejected fragments diminished very re<m- 



l, as the distance from 



same 



the centre of projection was greater. 



mass 



Herculaneum and Pompeii should be referred, has been a 

 question of the keenest controversy; but the discussion 

 might have been shortened, if the combatants had reflected 

 that, whether volcanic sand and ashes were conveyed to the 

 towns by running water, or through the air, during an erup- 

 tion, the interior of buildings, so long as the roofs remain 

 entire, together with all underground vaults and cellars, 

 could be filled only by an alluvium. We learn from history, 

 that a heavy shower of sand, pumice, and lapilli, sufficiently 

 great to render Pompeii and Herculaneum uninhabitable, 

 fell for eight successive days and nights in the year 79, ac- 

 companied by violent rains. We ought, therefore, to find a 



very 



close resemblance between the strata covering 



C5 



these 



towns and those composing the minor cones of the Phle- 

 grsean Fields, accumulated rapidly, like Monte Nuovo, during 

 a continued shower of ejected matter ; with this difference, 

 however, that the strata incumbent on the cities would be 

 horizontal, whereas those on the cones are highly inclined ; 



ulai 



e thrown 



out near the vent, would be wanting at a distance where 

 small lapilli only can be found. Accordingly, with these ex- 

 ceptions, no identity can be more perfect than the form and 

 distribution of the matter at the base of Monte Nuovo, as 

 laid open by the encroaching sea, and the appearance of the 



>osed on Pompeii. That city is covered with 

 numerous alternations of different horizontal beds of tuff' and 

 lapilli, for the most part thin, and subdivided into very fine 



im 



VOL. I. 



* Ed. Journ. of Science, No.xix. p. 131. Jan. 1829 



T T 



