cramped the freedom of our speculations in this
science, very much in the same way as a belief in the existence of a
vaulted firmament once retarded the progress of astronomy. It was not
until Descartes assumed the indefinite extent of the celestial spaces,
and removed the supposed boundaries of the universe, that just opinions
began to be entertained of the relative distances of the heavenly
bodies; and until we habituate ourselves to contemplate the possibility
of an indefinite lapse of ages having been comprised within each of the
modern periods of the earth's history, we shall be in danger of forming
most erroneous and partial views in geology.

If history had bequeathed to us a faithful record of the eruptions of
Etna, and a hundred other of the principal active volcanoes of the
globe, during the last three thousand years,--if we had an exact account
of the volume of lava and matter ejected during that period, and the
times of their production,--we might, perhaps, be able to form a correct
estimate of the average rate of the growth of a volcanic cone. For we
might obtain a mean result from the comparison of the eruptions of so
great a number of vents, however irregular might be the development of
the igneous action in any one of them, if contemplated singly during a
brief period.

It would be necessary to balance protracted periods of inaction against
the occasional outburst of paroxysmal explosions. Sometimes we should
have evidence of a repose of seventeen centuries, like that which was
interposed in Ischia, between the end of the fourth century B.C., and
the beginning of the fourteenth century of our era.[580] Occasionally a
tremendous eruption, like that of Jorullo, would be recorded, giving
rise, at once, to a considerable mountain.

If we desire to approximate to the age of a cone such as Etna, we ought
first to obtain some data in regard to the thickness of matter which has
been added during the historical era, and then endeavor to estimate the
time required for the accumulation of such alternating lavas and beds of
sand and scoriae as are superimposed upon each other in the Val del Bove;
afterwards we should try to deduce, from observations on other
volcanoes, the more or less rapid increase of burning mountains in all
the different stages of their growth.

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