44 



ETNA. 



1 



[Ch. XXVI. 



of such alternating lavas and beds of sand and scoriae as a 

 superimposed upon each other in the Val del Bove • after 

 wards we should try to deduce^ from observations on othe^ 

 volcanos^ the more or less rapid increase of burning' moun 

 tains in all the different stages of their growth. 



Although it is possible that some of the ancient eruptions 

 of which the products are seen in the walls of the Yal del 

 Bove were on as grand a scale as those of our own time 

 even grander, yet we should in vain seek for evidence that 

 any one of those ancient currents equalled in volume tte 

 lavas of 1669 or those of 1852. 



There is a considerable analogy between the mode of in- 

 crease of a volcanic cone and that of trees of exogenous growth. 

 These trees augment, both in height and diameter, by the 

 successive application externally of cone upon cone of new 



or 



ligneous matter ; so that if we 



m 



section 



near the base of the trunk, we intersect a much greater 



sum 



When 



occasionally shoot out from the trunk, they iirst pierce the 

 bark, and then, after growing to a certain size, if they 

 chance to be broken off, they may become inclosed in the 

 body of the tree, as it augments in size, formino- knots in 



themselves com 



matter 



manner, a volcanic mountain 



seen, of a succession 



masses 



enveloping others, 



while lateral cones, having a similar internal structure, oftei 

 project in the first instance, like branches from the surface 



mam 



fc) 



o 



like the knots of a tree. 



We can ascertain the age of an oak or pine by counting 

 the number of concentric rings of annual growth seen in a 

 transverse section near the base, so that we may know the 

 date at which the seedling began to vegetate. The Baobab- 

 tree of Senegal [Adansonia digitata) is supposed to exceed 

 almost any other in longevity. Adanson inferred that one 

 which he measured, and found to be thirty feet in diameter, 

 had attained the " 



incision to a certain depth, he first counted^ 300 



age of 5,150 years. 



Having made 



ail' 



rnig'S 



CH' 



^- 







f StB 







c; 



the 



iBore 



It 



resp^ 

 mate 



like] 



lava 



mom 



afion 



Eacl 



tine 



act: 

 rec 

 con 



■ side 



ToJca 



Matte 

 (le Be, 

 implyj 



of til 



e 



