SEQUEL TO SYSTEMATIC GEOLOGY. 529 



Hence those descriptive names only have betn really useful in geology 

 which had been used without any scrupulous regard to the appropri- 

 ateness of the description. The Green Sand may be white, brown, or 

 red ; the Mountain Limestone may occur only in valleys ; the Oolite 

 may have no roe-like structure ; and yet these may be excellent geo- 

 logical names, if they be applied to formations geologically identical 

 with those which the phrases originally designated. The signification 

 may assist the memory, but mnst not be allowed to subjugate the 

 'acuity of natural classification. 



The terms which have been formed by geologists in recent times 

 have been drawn from sources similar to those of the older ones, and 

 will have their fortune determined by the same conditions. Thus Mr. 

 Lyell has given to the divisions of the tertiary strata the appellation? 

 Pleiocene, Meiocene, JZocene, accordingly as they contain a majority of 

 recent species of shells, a minority of such species, or a small propor- 

 tion of living species, which may be looked upon as indicating the * 

 dawn of the existing state of the animate creation. But in this case, 

 he wisely treats his distinctions, not as definitions, but as the marks of 

 natural groups. " The plurality of species indicated by the name pleio- 

 cene, must not," he says,* " be understood to imply an absolute majority 

 of recent fossil shells in all cases, but a comparative preponderance 

 wherever the pleiocene are contrasted with strata of the period imme- 

 diately preceding." 



Mr. Lyell might have added, that no precise percentage of recent 

 species, nor any numerical criterion whatever, can be allowed to over- 

 bear the closer natural relations of strata, proved by evidence of a 

 superior kind, if such, can be found. And this would be the proper 

 answer to the objection made by De la Beche to these names ; namely, 

 that it may happen that the meiocene rocks of one country may be of 

 the same date as the pleiocene of another ; the same formation having 

 in one place a majority, in another a minority, of existing species. 

 We are not to run into this incongruity, for we are not so to apply the 

 names. The formation which has been called pleiocene, must continue 

 to be so called, even where the majority of recent species fails ; and all 

 rocks that agree with that in date, without further reference to the 

 numerical relations of their fossils, must also share in the name. 



To invent good names for these large divisions of the series of strata, 

 is indeed extremely difficult. The term Oolite is an instance in which 



4 Gcol. iii. 392. 

 VOL. II. 34 



