BREAKS IN THE GEOLOGICAL RECORD. 49 



beds of the older series (fig. 18) ; and a moment's considera- 

 tion will show us what this indicates. It indicates, beyond 



Fig. 18. Section showing strata of Tertiary age (a) resting upon a worn and eroded 

 surface of White Chalk (b), the stratification of which is marked by lines of flint. 



the possibility of misconception, that there was an interval, 

 between the deposition of the older series and that of the 

 newer series of strata ; and that during this interval the older 

 beds were raised above the sea-level, so as to form dry land, 

 and -were subsequently depressed again beneath the waters, to 

 receive upon their worn and wasted upper surface the sedi- 

 ments of the later group. During the interval thus indicated, 

 the deposition of rock must of necessity have been proceeding 

 more or less actively in other areas. Every unconformity, 

 therefore, indicates that at the spot where it occurs, a more or 

 less extensive series of beds must be actually missing ; and 

 though we may sometimes be able, to point to these missing 

 strata in other areas, there yet remains a number of unconfor- 

 mities for which we cannot at present supply the deficiency 

 even in a partial manner. 



It follows from the above that the serie> of stratified deposits 

 is to a greater or less extent irremediably imperfect ; and in 

 this imperfection we have one great cause why we can never 

 obtain a perfect series of all the animals and plants that have 

 lived upon the globe. Wherever one of these great physical 

 gaps occurs, we find, as we might expect, a corresponding 

 break in the series of life-forms. In other words, whenever we 

 find two formations to be unconformable, we shall always find 

 at the same time that there is a great difference in their fossils, 

 and that many of the fossils of the older formation do not sur- 

 vive into the newer, whilst many of those in the newer are not 

 known to occur in the older. The cause of this is, obviously, 



