PREFACE. XVII 



must select certain normal groups which do not 

 approximate too closely to each other, and enumerate 

 by name the species common to more than one of 

 these. Thus, for example, we might omit in our 

 tables the Newer Pliocene formations altogether, and 

 enumerate the shells common to the Recent and Older 

 Pliocene beds. 



I have arranged the tertiary formations in four 

 groups, as I had determined to do before I was 

 acquainted with M. Deshayes; and in his tables he 

 has referred the shells to three periods, according to 

 which he had classed them before he had any commu- 

 nication with me. No confusion, however, will arise 

 from this want of conformity between the tables and 

 my classification, since I have named two of my 

 periods (the Newer and Older Pliocene) as subdivisions 

 of one of his ; and by reference to the Synoptical 

 Table, at p. 61, the reader will see which localities 

 mentioned in M. Deshayes's Tables belong to the 

 Newer and which to the Older Pliocene period. 



In the summer of 1831 I made a geological 

 excursion to the volcanic district of the Eifel, and on 

 my return I determined to extend my work to three 

 volumes, the second of which appeared in January, 

 1832. The last volume has been delayed till now by 

 many interruptions, among which I may mention a 

 tour, in the summer of 1832, up the valley of the 

 Rhine, when I examined the loess (vol. iii. p. 151), 



VOL. III. c 



