22 INTRODUCTION. 



or less extent ; in addition, there would be a certain proportion 

 of forms of life wholly unknown in the Cretaceous Rocks ; and, 

 lastly, there would be a conspicuous absence of certain charac- 

 teristic species of the Chalk period. In other words, such 

 deposits as we have been speaking of would contain an assem- 

 blage of fossils more or less intermediate in character between 

 those of the true Cretaceous period and those of the lowest 

 Tertiary beds (Eocene), which rest upon the Chalk, or they 

 would present an intermixture of Cretaceous with Eocene 

 types. In point of fact, we have fragments of such interme- 

 diate deposits (in the Maastricht beds of Holland, the Pisolitic 

 Limestone of France, the Faxoe Limestone of Denmark, and 

 the Thanet Sands of Britain), and we find in them traces of 

 such an intermixture. 



We may pause here to consider how it is that we may never 

 hope to find a complete series of deposits linking on one great 

 formation to another, as, for example, the Chalk to the Eocene 

 Rocks. In the first place, only a limited portion of the earth 

 has as yet been properly examined, and we have therefore no 

 right to expect that we have as yet hit upon the area, or areas, 

 to which the process of rock-forming was transferred at the 

 close of the Cretaceous period proper in Europe. We have, 

 however, the full right to expect that we shall ultimately find 

 formations which will have to be intercalated in point of time 

 between the White Chalk and the Eocene ; and, as before 

 said, traces of such are already known to us. In the second 

 place, we have every reason to suppose that many of these 

 intermediate deposits have been destroyed at some period sub- 

 sequent to their formation by what is technically called 

 " denudation," or, in other words, by the action of rain, rivers, 

 ice, and the sea. In the third place, many of the missing 

 deposits *may have been concealed since their formation by 

 the deposition upon them of other newer rocks ; or they may 

 be situated in areas which are at present covered by the 

 ocean. Lastly, we must not forget that there may have been 

 times in which great changes in life were actively progressing 

 in areas in which there might be little or no contemporaneous 

 deposition of rock, so that the extreme terms of a series might 

 be preserved to us whilst all the intermediate links might have 

 escaped record. 



From these and similar causes, it is almost certain that we 

 shall never be able to point to a complete series of deposits 

 linking one great geological period, such as the Cretaceous, to 

 another, such as the Eocene. Still, we may well have a strong 

 conviction that such deposits must exist, or must have existed, 



