112 Geography, Geology. 
of corn, and is distinguished, even where uncultivated, by the 
luxuriance of the plants that grow upon it. This formation, 
though sometimes having a more arenaceous character, occurs 
along the western écaet from ‘Trapani to Sciacca; and a 
brecci ia of the same kind replete with shells, not far, if at all, 
removed from existing species, seems to fill up the hollows in 
most of the older rocks: of Sicily. It exists at Messina, at 
Syracuse, from whence it pr oceeds along the shore in the di- 
rection of Catania, near Castro-Giovanni, and Girgenti, &c. 
Dr. Daubeny does not decide whether the breccia found on the 
hills in the interior of the island, is the same as that on the 
coast between Trapani and Selinunte, but the character of the 
rock as well as its embedded fossils appear to coincide. 
The stratum on which this reposes is by far the most con- 
siderable in Sicily. Indeed, nearly half the surface of the 
island is constituted of this and the subordinate beds; as it 
extends from the neighbourhood of Palermo and ‘Termini on 
the north, to Terra Nuova on the south, occupies nearly the 
whole of the centre, and proceeds on the east to the skirts of 
Etna. The predominating rock in this formation is a bluish 
plastic clay, with which are associated beds of gypsum, and 
masses of eset of blue limestone, of a dark brown slaty 
marl, of a white argillaceous limestone, frequently alternating 
with marl, and of a brecciated calcareous rock, with oval frag- 
ments of a white compact limestone. The phe clay rarely 
contains shells; it possesses crystals of sulphate of lime, of 
sulphate of strontian, and of native sulphur, rock-salt, alum, 
sulphate of barytes, copper pyrites, and iron. 
The hill Macalubba near Girgenti is of blue clay, it is called 
the mud or air volcano, hecause: at times it emits a quantity of 
gas, and throws up muddy water to a considerable height. A 
similar chemical action takes place in the Monte di S. Calogero 
behind Sciacca, where at its summit hot vapours * continually 
issue from numerous crevices and clefts. At its base are hot 
sulphureous baths, situated in the blue clay, but the mountain 
itself is a white saccharoid limestone of a compact nature, con- 
taining flint and shells. The blue clay formation the professor 
believes to be of a very recent date, belonging probably to the 
tertiary epoch, and is not related to the new red or muria- 
tiferous sandstone of the north of Europe. 
A series of tertiary rocks occupy the southern portion ag 
the island, extending from Cape Passaro to the Lake Lentini, 
5 . . . 
where they are interrupted by a diluvial tract, called the plain 
* It is also singular that the same phenomena occur in the mountains of 
Pantellaria, which i is about seventy Italian miles distant to the south-west of 
Sciacca. That island is, according to Ferrara, altogether volcanic. 
