TWO ANTAGONIST DOCTRINES OF GEOLOGY. 665 



inhabiting the Mediterranean ; and soon after, ]}e 

 made collections of a similar description on the 

 flanks of Etna, in the Val di Noto, and in other 

 places. 



The impression produced by these researches is 

 described by himself 7 . " In the course of my tour 

 I had been frequently led to reflect on the precept 

 of Descartes, that a philosopher 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 surprize on visiting 

 Sortino, Pentalica, Syracuse, and other parts of the 

 Val di Noto, at beholding a limestone of enormous 

 thickness, filled with recent shells, or sometimes 

 with mere casts of shells, resting on marl in which 

 shells of Mediterranean species were imbedded in 

 a high state of preservation. All idea of [neces- 

 sarily] attaching a high antiquity to a regularly- 

 stratified limestone, in which the casts and impres- 

 sions of shells alone were visible, vanished at once 

 from my mind. At the same time, I was struck 

 with the identity of the associated igneous rocks of 

 the Val di Noto with well-known varieties of 'trap' 

 in Scotland and other parts of Europe ; varieties 

 which I had also seen entering largely into the 

 structure of Etna. 



" I occasionally amused myself," Mr. Lyell adds, 

 "with speculating on the different rate of progress 

 which geology might have made, had it been first 



7 Lyell, 1st ed, Pref. x. 



