in height and diameter, by the successive 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 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, 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, consists 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.

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
calculation made according to a supposed mean rate of increase. De
Candolle considers it not improbable that the celebrated Taxodium of
Chapultepec, in Mexico (_Cupressus disticha_, Linn.), which is 117 feet
in circumference, may be still more aged.[581]

It is, however, impossible, until more data are collected respecting the
average intensity of the volcanic action, to make any thing like an
approximation to the age of a cone like Etna; because, in this case, the
successive envelopes of lava and scoriae 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 scoriae, differing in quantity, and
which may have been accumulated in unequal periods of time. Yet we
cannot fail to form the most exa