Ch.VIIL] 



MODE OF INCREASE OF VOLCANOS. 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 (Cupressus 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 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 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. 



H3 



