AFFECTING THE PIL1NG-T/P OF VOLCANIC CONES. 745 



length to that of the embankment, and often fissured or cracked 

 transversely by deep clefts. Again, the extent to which the ground 

 beneath the foundation of ponderous architectural structures, such 

 as cathedral-towers, has been known to . become compressed, is as 

 remarkable as it is instructive and curious. The amount of de- 

 pression in some cases may be measured by feet ; and although the 

 foundation may be laid in material of great apparent uniformity, its 

 resistance to superincumbent weight when sufficient to cause dis- 

 placement may prove far from uniform. Thus the Campanile, or 

 leaning tower of Pisa, and the great square brick tower (only the 

 lower half of which remains) at Bologna (the upper half having fallen 

 off), have had the ground beneath each of them compressed un- 

 equally, and at one side to the extent of several feet*. Yet the 

 extreme pressure imposed on the unit of surface of any architectural 

 or engineering structure upon our globe is but a feather-weight as 

 compared with the pressure per unit of surface beneath the base of 

 any volcanic or other mountain of even moderate height. 



While the controversy raged between the advocates of " craters of 

 elevation," led by Yon Buch and Elie de Beaumont, and those who 

 with Lyell expressed the more rational notion, which may now be 

 considered established, that volcanic cones and their craters are 

 consequences of deposition, the advocates of those respective systems 

 were much more occupied with finding arguments in favour of their 

 own, and against the views of their opponents, than with carefully 

 examining the actual facts in the structure of volcanic cones ; and 

 hence the important fact of the change in position and of level in 

 the bases of volcanic cones during the long periods occupied by their 

 deposition, has, so far as my knowledge goes, hitherto entirely 

 escaped the notice of geologists. Yet the phenomenon, although 

 frequently difficult of observation, must be of such frequent occur- 

 rence as to deserve the prominence and explanation here sought to 

 be given to it. 



In addition to the cause which has been already referred to, for 

 the alteration of direction and position in the lower beds and be- 

 neath the bases of volcanic cones, namely the effect of mere weight 

 in forcing downwards the lower part of the mountain, another ad- 

 juvant cause must very frequently, if not always, come into play, 

 producing depression and alteration of slope in the lower parts and 

 beneath the base — namely, the excavation and honeycombing and 

 evisceration which is continually going on beneath such mountains. 

 Where such excavation takes place at considerable distances from 



* The Campanile or Leaning Tower of Pisa, built in the 12th century, is 

 cylindrical and abont 150 feet in height ; and the dense clay upon which it has 

 been founded has compressed unequally, so that the tower at one side over- 

 hangs its base by about 14 feet. The two square pyramidal towers of brick at 

 Bologna were also built in the 12th century ; the higher and nearly complete 

 one, usually called II Torre d'Asinelli, after the name of the founder, is 256 feet 

 in height, and stands nearly plumb ; the other, called Garisenda, is only about 

 half that height, and leans over to one side so that the summit overhangs the 

 base by about 9 feet. The latter tower is referred toby Dante in the 31st canto 

 of the ' Inferno.' 



