OCR Text |
Show xvi PREFACE. only occur in the Eocene period to which the strata of those basins belong. The names thus added will increase the value of the tables, and give a more complete view of the point to which fossil conchology has now reached ; at the same time, it must be admitted that tables of shells cannot be perfected on this plan, . as the science advances from year to year, without soon outgrowing the space which could reasonably be allotted to fossil conchology in a work on geology, for they would soon embrace the names of the greater number of known shells, nearly all of these being common to different groups of strata of the same period. Some of the catalogues which I have given in Appendix II., of fossil shells from the neighbourhood of the Red Sea, and from some other localities, may illustrate this remark, as they lead us to anticipate that, at no distant time, we may find a large proportion of all the Recent species in a fossil state. In treatises on fossil conchology, such as I trust M. Deshayes will soon publish, we cannot have too complete a catalogue of all the species which have been found fossil in every locality, together with their synonyms; but in geological works we can only ill us· trate the more important theoretical points by catalogues of those shells which are either characteristic of particular periods, as being exclusively confined to them, or which show the connexion of two periods, by being common to each. For this purpose we 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 communication 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 0 lder Pliocene) as subdivisions of one of his ; and by reference to the Synaptical Table, at p. 61, the reader will see which localities mentioned in M. Desha yes'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 |