Its aggregate thickness amounts sometimes to
700 or 800 feet. It usually occurs in regular horizontal beds, and is
occasionally intersected by deep valleys, such as those of Sortino and
Pentalica, in which are numerous caverns. The fossils are in every stage of
preservation, from shells retaining portions of their animal matter and colour
to others which are mere casts. The limestone passes downward into a sandstone
and conglomerate, below which is clay and blue marl, from which perfect shells
and corals may be disengaged. The clay sometimes alternates with yellow sand.
South of the plain of Catania is a region in which the tertiary beds are
intermixed with volcanic matter, which has been for the most part the product of
submarine eruptions. It appears that, while the clay, sand, and yellow limestone
before mentioned were in course of deposition at the bottom of the sea,
volcanoes burst out beneath the waters, like that of Graham Island, in 1831, and
these explosions recurred again and again at distant intervals of time. Volcanic
ashes and sand were showered down and spread by the waves and currents so as to
form strata of tuff, which are found intercalated between beds of limestone and
clay containing marine shells, the thickness of the whole mass exceeding 2000
feet.
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