64 HISTORY OF GEOLOGY. 



plored by M. Constant Prevost. Newer and higher 

 still than these, are found the Sub-Apennine forma- 

 tions of Northern Italy, and probably of the same 

 period, the English " crag" of Norfolk and Suffolk. 

 And most of these marine formations are associated 

 with volcanic products and fresh-water deposits, so 

 as to imply apparently a long train of alternations 

 of corresponding processes. It may easily be sup- 

 posed that, when the subject had assumed this form, 

 the boundary of the present and past condition of 

 the earth was in some measure obscured. But it 

 was not long before a very able attempt was made 

 to obliterate it altogether. In 1828, Mr. Lyell set 

 out on a geological tour through France and Italy". 

 He had already conceived the idea of classing the 

 tertiary groups by reference to the number of 

 recent species which were found in a fossil state. 

 But as he passed from the north to the south of 

 Italy, he found, by communication with the best 

 fossil conchologists, Borelli at Turin, Guidotti at 

 Parma, Costa at Naples, that the number of extinct 

 species decreased ; so that the last-mentioned natu- 

 ralist, from an examination of the fossil shells of 

 Otranto and Calabria, and of the neighbouring seas, 

 ; was of opinion that few of the tertiary shells were 

 of extinct species. To complete the series of proof, 

 Mr. Lyell himself explored the strata of Ischia, and 

 found, 2000 feet above the level of the sea, shells, 

 .which were all pronounced to be of species now 



c 1st eel. vol. iii. Pref. 



