Ch. VIII.] MODE OP INCREASE OF VOLCAHOS. 
99 
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 age of 5150 years. Having made an incision 
to a certain depth, he first counted three hundred rings 
of annual growth, and observed what thickness the tree had 
gained in that period. The average rate of growth of younger 
trees, of the same species, was then ascertained, and the calcu- 
lation, made according to a supposed mean rate of increase. 
De Candolle considers it not improbable, that the celebrated 
Taxodium of Chapultepec, in Mexico (Ciqiressus disticha, 
Linn.), which is one hundred and seventeen feet in circum- 
ference, may be still more aged*. 
It is, however, impossible, until more data are collected 
respecting the average intensity of the volcanic action, to make 
anything like an approximation to the age of a cone like Etna, 
because, in this case, the successive envelopes of lava and scorias 
are not continuous, like the layers of wood in a tree, and afford 
us no definite measure of time. Each conical envelope is made 
up of a great number of distinct lava-currents and showers of 
sand and scoria?, differing in quantity, and which may have 
been accumulated in unequal periods of time. Yet we cannot 
fail to form the most exalted conception of the antiquity of this 
mountain, when we consider that its base is about ninety miles 
in circumference ; so that it would require ninety flows of lava, 
each a mile in breadth at their termination, to raise the present 
foot of the volcano as much as the average height of one lava- 
current. 
There are no records within the historical era which lead 
to the opinion, that the altitude of Etna has materially varied 
* On the Longevity of Trees, Bibliot. Univ., May, 1831. 
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