50 PRINCIPLES OF PALAEONTOLOGY. 



from one great formation to another, but that, on the contrary, 

 the younger formation rests " unconformably, " as it is called, 

 either upon the formation immediately preceding it in point of 

 time, or upon some still older one. The essential physical 

 feature of this unconformability is that the beds of the younger 

 formation rest upon a worn and eroded surface formed by the 

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

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



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. 



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 sediments 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 unconformities for which we cannot at present supply 

 the deficiency even in a partial manner. 



It follows from the above that the series 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 



