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Ch. XXV.] 



TO MONTE SOMMA OR TO VESUVIUS. 



635 



The ravine-like character of these c barrancos 5 is due, I 

 conceive, to their having been excavated by torrents cutting 

 their way backwards. In a neighbouring valley, namely, 

 that of Olivelli, are seen trachytic lavas with pumiceous 

 tuffs like those which cover Pompeii, but of much older 

 date. Blocks of white dolomite, sometimes more than a foot 

 in diameter, occur, though rarely in the ancient tuffs, as 

 well as blocks of lava four or five feet in diameter. Above 

 the village of Somma we examined the parallel and adjoining 

 ravines called the Yallone di Panico and the Vallone di Cas- 

 tello. Although they are very near each other, the dissimi- 

 larity of the sections which they present to the geologist is 

 very marked. It is the same everywhere, the greatest diver- 

 sity of character prevailing in the details of structure instead 

 of that uniformity, for explaining which the theory of up- 

 heaval or elevation craters was invented. Thus, on the right 

 side of the ravine called the Casa delP Acqua is seen a pink 

 lava more than 30 feet thick, containing crystals of augite 

 and leucite, and inclined at an angle of 20 degrees, to which 

 there is nothing answering in the next closely adjoining 

 valleys to the east and west. The same dense mass terminates 

 abruptly at its lower end. Some of the lavas and tuffs in 

 the same ravine are unconformable to other sets, and must 

 have flowed down after valleys had been excavated in the 

 flanks of the old cone. The entire absence of dikes in most 

 of the valleys, even in those upper parts of them which are 

 only separated by a distance of a few hundred yards from the 

 section in the Atrio where the dikes are so abundant, would 

 have surprised me had I not been familiar with the same 

 phenomenon in other volcanic mountains, especially the 

 Canaries, where the dikes are almost entirely confined to the 

 vicinity of the grand centres of eruption. 



The annexed section (fig. 68) may give some idea of the 

 general character of the north of Somma, so far as it is pos- 

 sible to represent in one view the structure of a cone, the 

 separate parts of which are so unlike each other. The dikes 

 so conspicuous in the Atrio terminate soon after the crest a ; 

 from a to b a great thickness of stony and scoriaceous lava, 

 dipping at angles of about 20 degrees, is most conspicuous ; 



