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Show • 98 NEWER PLIOCENE PERIOD· [Ch. VIII. It would be necessary to ba1 a nce pro t r acted periods of. in. action aO'a.m st the occas.w na1 ou tb ur.s t of paroxysmal explosiOns. • 0 Somet1mes we shoulc1 h ave evi·c 1 c nce 0 f a• repos•e of seventeen centuri.e s, 1.1 1 t lli'ch was interposed m Ischia, between the 1 {e t 1a w . . en d 0 f t l1 e !JO! Ul' tl1 ce,n tL1ry ' n • c ., and the begmnmg of the four- teent 11 cen t ury of Our era ;f.: · Occasionally a tremendous erup-tion, like that of Jorullo, would be ~corded, giving rise, at once, to a considerable mountain. If we desire to approximate to the age of a cone such as Etna we ouo·ht first to obtain some data in regard to the thick~ess of 0 matter which has been added during the historical era, and then endea,·our to estimate the time required for the accumulation of such alternating lavas and beds of sand and sc01·ire as are superimposed upon each other in the Val del Bove ; afterwards we should try to deduce, from observations 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 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 ~ut 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 vol~~nic 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. xix, Ch. VIII.] MODE OF INCREASE OF VOLCANOS, 99 We can ascertain the age of an oak or pine, by counting the number of concentric rings of annu.al 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 measm·ed, 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 1·ings of annual growth, and observed what thickness the tree had gained in that period. The average rate of growth of younger tl'ees, 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 'l'axodimn of Chapultepec, in Mexico ( Cupressus disticha, Linn.), which is one hundred and seventeen feet in circumference, may be still more aged*. It is, however, impossible, until more data are co11ectecl 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 scm·i~ are not continuous, like the layers of wood in a tree and afford us no definite measure of time. Each conical cnvcl~pe is made up of a great number of distinct Java-currents and showers of sand and scoria>, ~iffering in quantity, and which may have b~en accumulated In unequal periods of time. y ct we cannot fall to f~rm the most exalted conception of the antiquity of this ~o~ntmn, when we consider that its base is about uinety miles In CH'cum.fer:nce; so that it would require ninety flows of lava, each a mtle m breadth at their termination, to raise the present foot of the volcano as much as the average height of one lava. current. There .a~e 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. H2 |