138 THE GROWTH OF CONTINENTS. 
sets of strata, resulting in the establishment by 
D’Orbigny of a still greater number of periods, 
marked by the successive deposits of the Jurassic 
and Cretaceous seas, all of which contained dif- 
ferent organic remains. The attention of geolo- 
gists being once turned in this direction, the 
other epochs were studied with the same view, 
and all were found to be susceptible of division 
into a greater or less number of such periods. 
I have dwelt at greater length on the Jurassic 
and Cretaceous divisions, because I believe that 
we have in the relation of these two epochs, as 
well as in that of the Cretaceous epoch with the 
Tertiary immediately following it, facts which 
are very important in their bearing on certain 
questions, now loudly discussed, not only by sci- 
entific men, but by all who are interested in the 
mode of origin of animals. Certainly, in the in- 
land seas of the Cretaceous and subsequent Ter- 
tiary times, where we can trace in the same sheet 
of water not only the different series of deposits 
belonging to two successive epochs in immediate 
juxtaposition, but those belonging to all the pe- 
riods included within these epochs, with the or- 
ganic remains contained in each, — there, if any- 
where, we should be able to trace the transition- 
types by which one set of animals is said to have 
been developed out of the preceding. We hear 
a great deal of the interruption in geological de- 
posits, of long intervals, the record of which has 
