C22 



STRUCTURE OF THE CONE OF VESUVIUS. 



[Ch. 



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winds in particular eruptions, and the small breadth of each 

 sheet of lava as it first flows out from the crater. 



But on a closer examination, we find that the appearance 

 of extreme uniformity is delusive ; for when a number of 

 beds thin out gradually, and at different points, the eye does 

 not without difficulty recognise the termination of any one 

 stratum, but usually supposes it continuous with some other, 

 which at a short distance may lie precisely in the same plane. 

 The difficulty, moreover, of following any given layer is in- 

 creased by its undulating form, produced by the moulding of 

 successive layers on the outer sides of a cone, which can 

 never preserve perfect symmetry owing to its irregular mode 

 of growth. As countless beds of sand and scorise constitute 

 the o-reater part of the whole mass, these may sometimes 

 mantle continuously round the whole cone ; and even lava 

 streams may be of considerable breadth when first they over- 

 flow, and since, in some eruptions, a considerable part of the 

 upper portion of the cone breaks down at once, may form a 

 sheet extending as far as the space which the eye usually 

 takes in, in a single section. 



The high inclination of some of the beds, and the firm 

 union of the particles even where there is evidently no ce- 

 ment, is another striking feature in the volcanic tuffs and 

 breccias, which seems at first not very easy of explanation. 



am 



the manner in which these strata are formed. Fragments of 

 lava, scoriae, pumice, and sand, when they fall at slight dis- 

 tances from the summit, are only half cooled down from a 

 state of fusion, and are afterwards acted upon by the heat 

 from within, and by fumeroles or small crevices in the cone 

 through which hot vapours are disengaged. Thus heated, 



the ejected 



fragments cohere together 



strongly; and the 



whole mass acquires such consistency in a few days, that 

 fragments cannot be detached without a smart blow of the 

 hammer. At the same time sand and scoriae, ejected to a 

 greater distance, remain incoherent.* 



Sir William Hamilton, in his description of the eruption of 

 1779, says, that jets of liquid lava, mixed with stones and 



* Monticelli and Covelli, Storia di Fenon. del. Vesuv. in 1821-23. 









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