98 
NEWER PLIOCENE PERIOD. 
[Ch. VIII. 
It would be necessary to balance protracted periods of in- 
action against the occasional outburst of paroxysmal explosions. 
Sometimes we sliould have evidence of a repose of seventeen 
centuries, like that which was interposed in Ischia, between the 
end of the fourth century, b. c, and the beginning of the four- 
teenth century of our era *. Occasionally a tremendous erup- 
tion, like that of Jorullo, would be recorded, giving rise, at 
once, to a considerable mountain. 
If we desire to approximate to the age of a cone such as 
Etna, we ought first to obtain some data in regard to the 
thickness of matter which has been added durino; the his- 
torical era, and then endeavour to estimate the time required 
for the accumulation of such alternating lavas and beds of 
sand and scorise as are superimposed upon each other in the 
Val del Bove ; afterwards we should try to deduce, from ob- 
servations on other volcanos, the more or less rapid increase of 
burning mountains in all the different stages of their growth. 
Mode of increase of volcanos analogous to that of exogenous 
trees.' — There is a considerable analogy between the mode of 
increase of a volcanic cone and that of trees of exogenous growth. 
These trees augment, both in height and diameter, by the suc- 
cessive application externally of cone upon cone of new ligneous 
matter, so that if we make a transverse section near the base of 
the trunk, we intersect a much greater number of layers than 
nearer to the summit. When branches occasionally shoot out 
from the trunk they first pierce the bark, and then, after grow- 
ing 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, 
forming knots in the wood, which are themselves composed of 
layers of ligneous matter, cone within cone. 
In like manner a volcanic mountain, as we have seen, con- 
sists of a succession of conical masses enveloping others, while 
lateral cones, having a similar internal structure, often project, 
in the first instance, like branches from the surface of the main 
cone, and then becoming buried again, are hidden like the knots 
of a tree. 
* See vol, i, chap, six, 
