68 INTRODUCTION. 



the occurrence of individuals being killed whilst swimming a river 

 or some other piece of water, or being mired in a bog, or to 

 the bones of those that had died on land being washed into some 

 stream, and thence into a lake or into the sea, by floods ; but there 

 are other cases for which a different explanation must be sought. 

 The most abundant remains of Mammals have been found in de- 

 posits which have been laid down in lakes. 



II. Unrepresented Time. — In the second place, we have seen 

 that the geological record is very imperfect, and this of necessity 

 causes vast gaps in our palseontological knowledge. In this con- 

 nection we may briefly consider the evidence which we possess as 

 to the immensity of the " unrepresented time " between some of the 

 great formations, and no better example can be chosen than that 

 of the Cretaceous and Eocene rocks, as developed in Europe. In 

 considering such a case, the evidence may be divided into two 

 heads, the one palaeontological, the other purely physical, and each 

 may be looked at separately. 



The Chalk, as is well known, constitutes in Britain the topmost 

 member of the Cretaceous formation, and is the highest deposit 

 there known as appertaining to the great Secondary or Mesozoic 

 series. It is directly overlaid in various places by strata of Eocene 

 age, which form the base of the great Tertiary or Kainozoic series 

 of rocks. The question, then, before us is this, What evidence have 

 we as to the lapse of time represented in Britain merely by the 

 dividing-line between the highest beds of the Chalk and the lowest 

 beds of the Eocene ? 



Taking the palaeontological evidence first, it is found that not a 

 single specific type out of the vast number of known Cretaceous 

 fossils has hitherto been recognised with certainty as occurring in 

 the immediately overlying Eocene beds. These latter, on the con- 

 trary, are replete with organic remains wholly distinct from those of 

 the Cretaceous beds. It may be said, therefore, that the very ex- 

 tensive assemblage of animals which lived in the later Cretaceous 

 seas of Britain had entirely passed away and become a thing of the 

 past, before a single grain of the Eocene rocks had been deposited. 

 Now it is of course open to us to believe that the animals of the 

 Chalk sea were suddenly extinguished by some natural agencies 

 unknown to us, and that the animals of the Eocene sea had 

 been in as sudden and as obscure a manner introduced en masse 

 into the same waters. This theory, however, calls upon the stage 

 forces of which we know nothing, and is contradicted by the whole 

 tenor of the operations which we see going on around us at the 

 present day. It is preferable, therefore, to believe that no such 

 violent processes of destruction and repeopling took place, but that 

 the marked break in the life of the two periods indicates an enor- 



