SECONDARY FORMATIONS. [Ch. XXIII. 



(jencra distinct from those established for the classification of 

 tertiary and recent fish. 



Chasm between the Eocene and Maestricht formations. 

 There appears, then, to be a greater chasm between the organic 

 remains of the Eocene and Maestricht beds, than between the 

 Eocene and Recent strata ; for there are some living shells in 

 the Eocene formations, while there are no Eocene fossils in the 

 newest secondary group. It is not improbable that a greater 

 interval of time may be indicated by this greater dissimilarity 

 in fossil remains. In the 3rd and -1th chapters we endeavoured 

 to point out that we have no right to expect, even when we 

 have investigated a greater extent of the earth's surface, that 

 we shall be able to bring to light an unbroken chronological 

 series of monuments from the remotest eras to the present ; 

 but as we have already discovered a long succession of deposits 

 of different ages, between the tertiary groups first known and 

 the recent formations, so we may, perhaps, hereafter detect an 

 equal, or even greater series, intermediate between the Maes- 

 tricht beds and the Eocene strata. 



Duration of secondary periods. The different subdivisions 

 of the secondary group No. 1, extending from the chalk of 

 Maestricht to the lower green-sand inclusive, may, perhaps, 

 relate to a lapse of ages as immense as the united tertiary 

 periods, of which we have sketched the eventful history in this 

 volume. Such a conjecture, at least, seems warranted, if we 

 can form any estimate of the quantity of time, by comparing 

 the amount of vicissitude in animal life which has occurred 

 during its lapse. 



Position of former continents. The existence of sea as 

 well as land, at every geological period, is attested by the re- 

 mains of terrestrial plants imbedded in the deposits of all ages, 

 even the most remote. We find fluviatile shells not un- 

 frequently in the secondary strata, and here and there some 

 fresh-water formations ; but the latter are less common than 

 in the tertiary series. Eor this fact we have prepared the 

 reader's mind, by the views advanced in the third chapter 



