328 
SECONDARY FORMATIONS. 
[Ch. XXIII. 
genera 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 4th 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. For this fact we have prepared the 
reader's mind, by the views advanced in the third chapter 
