206 
ELEMENTS OF GEOLOGY. 
in which are numerous caverns. The fossils are in every 
stage of preservation, from shells retaining portions of their 
animal matter and color to others which are mere casts. 
The limestone passes downward into a sandstone and con¬ 
glomerate, below which is clay and blue marl, from which 
perfect shells and corals may be disengaged. The clay some¬ 
times alternates with yellow sand. 
South of the plain of Catania is a region in which the ter¬ 
tiary 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 tuif, which are found inter¬ 
calated between beds of limestone and clay containing ma- 
rine shells, the thickness of the whole mass exceeding 2000 
feet. The fissures through which the lava rose may be seen 
in many places, forming what are called dikes. 
No sliell is more conspicuous in these Sicilian strata than 
the great scallop, (Fig. 133), now so common 
in the neighboring seas. The more we reflect on the pre¬ 
ponderating number of this and other recent shells, the more 
Fig. 133. 
Pecten jacohceus; half natural size. 
