The last-mentioned shell
had already become rare when the associated marine and volcanic strata above
alluded to were formed. On the whole, the modern character of the testaceous
fauna under consideration is expressed not only by the small proportion of
extinct species, but by the relative number of individuals by which most of the
other species are represented, for the proportion agrees with that observed in
the present fauna of the Mediterranean. The rarity of individuals in the extinct
species is such as to imply that they were already on the point of dying out,
having flourished chiefly in the earlier Pliocene times, when the Subapennine
strata were in progress.
Yet since the accumulation of these Newer Pliocene sands and clays, the whole
cone of Etna, 11,000 feet in height and about 90 miles in circumference at its
base, has been slowly built up; an operation requiring many tens of thousands of
years for its accomplishment, and to estimate the magnitude of which it is
necessary to study in detail the internal structure of the mountain, and to see
the proofs of its double axis, or the evidence of the lavas of the present great
centre of eruption having gradually overwhelmed and enveloped a more ancient
cone, situated 3 1/2 miles to the east of the present one.
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