X 
PREFACE. 
three exceptions, recognized by Signor Costa as 
species now inhabiting the Mediterranean, a circum- 
stance which greatly astonished me, as I procured 
some of them at the height of 2000 feet above the 
level of the sea (Vol. iii. p. 126). 
Early in November, 1828, I crossed from Naples to 
Messina, and immediately afterwards examined Etna, 
and collected on the flanks of that mountain, near 
Trezza, the fossil shells alluded to in the third volume 
(p. 79, and Appendix II., p. 53). The occurrence of 
shells in this locality was not unknown to the natu^- 
ralists of Catania, but having been recognized by 
them as recent species, they were supposed to have 
been carried up from the sea-shore to fertilize the 
soil, and therefore disregarded. Their position is 
well known to many of the peasants of the country, by 
whom the fossils are called 'roba di diluvio.' 
In the course of my tour I had been frequently led 
to reflect on the precept of Descartes, ' that a philo- 
sopher should once in his life doubt every thing he 
had been taught ; ' but I still retained so much faith 
in my early geological creed as to feel the most lively 
surprise, on visiting Sortino, Pentalica, Syracuse, and 
other parts of the Val di Noto, at beholding a lime- 
stone of enormous thickness filled with recent shells, 
or sometimes with the mere casts of shells, resting on 
marl in which shells of Mediterranean species were 
imbedded in a high state of preservation. All idea 
of attaching a high antiquity to a regularly stratified 
