Ch. VII.] 
SUB-ETNEAN FORMATIONS. 
77 
which the annexed section is drawn, and in several other dis- 
tricts where the ' creta ' prevails, together with associated sub- 
marine lavas, and where there is no limestone capping, a vol- 
cano should now burst forth, and give rise to a great cone, the 
position of such a cone would exactly correspond to that of the 
modern Etna, with relation to the rocks on which it rests. 
Southern base of Etna. — The marine strata of clay and sand 
already alluded to, alternate in thin layers at the southern base 
of Etna, sometimes attaining a thickness of three hundred feet, 
or more, without any intermixture of volcanic matter. Crystals 
of selenite are dispersed through the clay, accompanied by a 
few shells, almost entirely of recent Mediterranean species. 
This formation of blue marl and yellow sand greatly resembles 
in character that of the Italian Subapennine beds, and, like 
them, often presents a surface denuded of vegetation, in con- 
sequence of the action of the rains on soft incoherent materials. 
In travelling by Paterno, Misterbianco, and La Motta, we 
pass through deep narrow valleys excavated through these beds, 
which are sometimes capped, as at La Motta, by columnar 
basalt, accompanied by strata of tuff and volcanic conglome- 
rate. (Diagram No. 13.) 
No. 13. 
La Motta near Catania, 
The latter rock is composed of rolled masses of basalt, which 
may either have originated when first the lava was produced 
in a volcanic archipelago, or subsequently when the whole coun- 
try was rising from beneath the level of the sea. Its occur- 
rence in this situation is striking, as not a single pebble can be 
