162 



LYELL'S ELEMENTS OF GEOLOGY. 



Chronological Classification of Aqueous Rocks. 



Test by included fragments of old rocks. It was stated, 

 that independent proof may sometimes be obtained of the rela- 

 tive date of two formations, by fragments of an older rock being 

 included in a newer one. This evidence may sometimes be of 

 great use, where a geologist is at a loss to determine the relative 

 age of two formations, from want of clear sections exhibiting 

 their true order of position, or because the strata of each group 

 are vertical. In such cases we sometimes discover that the 

 more modern rock has been in part derived from the degradation 

 of the older. Thus, for example, we may find chalk with flints ; 

 and, in another part of the same country, a distinct series, con- 

 sisting of alternations of clay, sand, and pebbles. If some of 

 these pebbles consist of flints, with fossil shells of the same spe- 

 cies as those in the chalk, we may confidently infer that the chalk 

 is the oldest of the two formations. 



The number of groups into which the fossiliferous strata may 

 be separated, is more or less, according to the views of classifi- 

 cation which different geologists entertain; but when we have 

 adopted a certain system of arrangement, we immediately find 

 that a few only of the entire series of groups occur one upon 

 the other in any single section or district. 



The thinning out of individual strata was before described 

 (p. 31.). But let the annexed diagram represent seven fossili- 



Fig. 127. 



ferous groups, instead of as many strata. It will then be seen 

 that in the middle all the superimposed formations are present ; 

 but in consequence of some of them thinning out, No. 2. and 

 No. 5. are absent at one extremity of the section, and No. 4. at 

 the other. 



If the reader consults the Frontispiece, he will see, that as the 

 strata A rest unconformably upon the older groups, a, ft, c, e,f, 

 g, we should meet with a very different succession in a vertical 

 section exposed at different places ; in one spot A lying imme- 

 diately on c, in another on g, and so forth. Now here the dif- 

 ference has been partly occasioned by denudation ; the forma- 

 tions a, ft, for instance, once extended much farther to the left, 

 and but for denudation would have been everywhere interposed 

 between A and the rocks e, f, g. In many instances the entire 

 absence of one or more formations of intervening periods be- 

 tween two groups, such as A and c, (see Frontispiece,) arises, 



