Speaking of the confusion and havoc which a man would make of history, who crowded the events of two thousand years into the space of one hundred, he says:

"He who should study the monuments of the natural world, under the influence of a similar infatuation, must draw a no less exaggerated picture of the energy and violence of causes, and must experience the same insurmountable difficulty in reconciling the former and present state of nature. If we could behold in one view all the volcanic cones thrown up in Iceland, Italy, Sicily, and other parts of Europe, during the last five thousand years, and could see the lavas which have flowed during the same period; the dislocations, subsidences, and elevations caused by earthquakes; the lands added to the various deltas, or devoured by the sea, together with the effects of devastations by floods; and imagine that all these events had happened in one year, we must form the most exalted ideas of the activity of the agents, and the suddenness of the revolutions. Were an equal amount of change to pass before our eyes in the next year, could we avoid the conclusion that some great crisis of nature was at hand? If geologists then have misinterpreted the signs of a succession of events, so as to conclude that centuries were implied where the characters imported thousands of years, and thousands of years where the language of nature implied millions, they could not, if they reasoned logically, from such false premises, come to any other conclusion, than that the system of the world had undergone a complete revolution.

"We should be warranted in ascribing the erection of the Great Pyramid to superhuman power, if we were convinced that it was raised in one day; and if we imagine in the same manner a mountain chain to have been elevated during an equally small fraction of the time which was really occupied in upheaving it, we might then be justified in inferring that the subterraneous movements were once far more energetic than in our times. We know that one earthquake may raise the coast of Chili for one hundred miles to the average of five feet. A repetition of two thousand shocks of equal violence might produce a mountain-chain one hundred miles long, and one thousand feet high. Now should one only of these convulsions happen in a century, it would be consistent with the order of events experienced by the Chilians from the earliest times; but if the whole of them were to occur in the next hundred years, the entire district must be depopulated; scarcely any animals or plants could survive; and the surface would be one confused heap of ruin and desolation."*

After treating most extensively on the powers and effects of volcanic agency, and showing, by a variety of adduced instances, that the production of burning mountains, of any magnitude, is a slow and gradual process in the support of which line of argument his data are peculiarly strong and nervous -he states:-"There is a considerable degree of 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 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 enclosed 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 a 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 five thousand one hundred and fifty 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

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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 one hundred and seventeen feet in circumference, may be still more aged.

"It is, however, impossible, until more data are collected respecting the average intensity of volcanic action, to make any thing like an approximation to the age of a cone like Ætna; because in this case, the successive envelopes of lava and scoriæ are not continuous, like the layers of wood in a tree, and afford us no definite measure of time. Each conical envelop is made up of a great number of distinct lava-currents, and showers of sand and scoriæ, 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 æra, which lead to the opinion, that the altitude of Etna has materially varied within the last two thousand years. Of the eighty most conspicuous minor cones which adorn its flanks, only one of the largest, Monte Rossi, has been produced within the times of authentic history. Even this hill, thrown up in the year 1669, although four hundred and fifty feet in

height, only ranks as a cone of second magnitude. Monte Minardo, near Bronte rises, even now, to the height of seven hundred and fifty feet, although its base has been elevated by more modern lavas and ejections. The dimensions of these larger cones appear to bear testimony to paroxysms of volcanic activity; after which we may conclude from analogy that the fires of Etna remained dormant for many years, since nearly a century of rest has sometimes followed a violent eruption in the historical æra. It must also be remembered, that of the small number of eruptions which occur in a century, one only is estimated to issue from the summit of Ætna, for every two that proceed from the sides. Nor do all the lateral eruptions give rise to such cones as would be enumerated amongst the smallest of the eighty hills above enumerated; some produce merely insignificant monticules, soon destined to be buried, as we have before explained.

"How many years then must we not suppose to have been expended in the formation of the eighty cones? It is difficult to imagine that a fourth part of them have originated during the last thirty centuries. But if we conjecture the whole of them to have been formed in twelve thousand years, how inconsiderable an æra would this portion of time constitute in the history of the volcano. If we could strip off Etna all the lateral monticules now visible, together with the lavas and scoriæ that have been poured out from them, and from the highest crater during the period of their growth, the diminution of the entire mass would be ex tremely slight! Etna might lose perhaps several