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 



