20 INTRODUCTION. 



contain remains of the fresh-water and terrestrial animals of 

 the period, and of these only ; whilst the latter will principally, 

 if not exclusively, be characterised by the remains of marine 

 forms of life. In this way, there is reason to believe, may be 

 explained the differences between the fossils of the Old Red 

 Sandstone and of the Devonian Rocks, strictly so called. Both 

 are believed to have been deposited in the same geological 

 period, and to be truly " contemporaneous ; " but they do not 

 contain the same fossils. This may be readily explained, how- 

 ever, if we suppose the former to represent the fresh-water 

 deposits of the Devonian period, or to have been laid down in 

 an inland sea, whilst the latter is the true marine formation of 

 the same period. 



We are now in a position very briefly to discuss the question 

 of what may be called "geological continuity." It has already 

 been stated that the entire series of Fossiliferous or Sedi- 

 mentary Rocks may be naturally divided into a certain num- 

 ber of definite rock-groups or " formations," each of which is 

 characterised by the possession of a peculiar and characteristic 

 assemblage of fossils, constituting, or rather representing, the 

 "life" of the "period" in which the formation was deposited. 

 The older geologists held, what probably every one would be 

 tempted to think at first, that the close of each formation was 

 characterised by a general destruction of the forms of life of 

 the period, and that the commencement of each new formation 

 was accompanied by the creation of a number of new animals 

 and plants, destined to figure as the characteristic fossils of 

 the same. This theory, however, not only invokes forces and 

 processes which it can in no way account for, but overlooks 

 the fact that most of the great formations are separated by 

 lapses of time, unrepresented perhaps by any deposition of 

 rock, or represented only in some particular area, and yet, 

 perhaps, as great as, or greater than, the whole time occupied 

 in the production of the formation itself. 



Nowadays, most geologists hold that there was no such 

 sudden destruction of life at the close of each great geological 

 epoch, and no such creation of fresh forms at the commence- 

 ment of the next period. On the contrary, they hold that 

 there is a geological "continuity," such as we see in other 

 departments of nature, and that the lines which we draw 

 between the great formations merely mark periods of time in 

 which no rocks were laid down, or the rocks deposited in 

 which are at present unknown to us. 



What are we to believe occurred at the close of any great 

 geological period say, the Cretaceous period? If we reject 



