88 



In examining recent deposits within the tropics, we often find 

 the accumulations of only a particular variety of plants : and it is 

 remarkable that some plants which are common in the neigh- 

 bourhood are yet absent in the deposits. Some appear more 

 susceptible to rapid decay than others ; hence it follows that if 

 a series of beds should be found to contain only a particular 

 class of plants, it is no reason but that other kinds grew, not only 

 in the region but within the district. We note these facts in 

 order to guard against the too hasty conclusions of supposing 

 that the fossils always represent the real character, number and 

 variety, of the living organic system during their deposition. 



4. North Tropical Zone. — The European, Cretaceous and Tertiary 



Deposits. 



Although the English sedimentary rocks are divided into di- 

 stinct groups, it is not a natural and definite subdivision, but 

 depending entirely on local circumstances. A perfect transition 

 from one bed into another is of more common occurrence in 

 nature than distinct separation. We have no cretaceous equiva- 

 lent in the equatorial zone ; nor can we expect to find such a 

 deposit, if as we suppose, this is a marine formation, formed 

 in that part of the globe. No remains of mammiferous animals 

 have been discovered in the cretaceous series of England, and the 

 exuviae of reptiles are by no means common, showing that the 

 deposit was formed somewhat distant from dry lands. The re- 

 mains of a crocodile have, however, been found in the chalk of 

 Meudon, and the exuviae of a large reptile, the Mososaurus, at 

 Maestricht. The remains of fish are not so rare. The supracre- 

 taceous deposits are commonly termed tertiary, which denomi- 

 nation is exceedingly objectionable and inapplicable, as it implies 

 that there were three classes of rocks possessing marked charac- 

 teristic distinctions, and that the deposits above the chalk con- 

 stituted the third of such classes*. We should rather consider, 

 that as other groups pass into each other in one place, while 

 there is evidently a geological break between them in another, 

 that the like should happen with respect to the cretaceous and 

 supracretaceous groups. 



In examining the deposits which are now forming within the 

 tropical region, we often find a great number of beds containing 



* De la Beche's Geological Researches, p. 355. 



