er locality for the study of extinct forms of life than the
Jura. In all its breaks and ravines, wherever the inner surface of the
rock is exposed, it is full of organic remains; and to take a handful of
soil from the road-side is often to gather a handful of shells. It is
actually built of the remains of animals, and there are no coral reefs in
existing seas presenting a better opportunity for study to the naturalist
than the coral reefs of the Jura. Being already tolerably familiar with
the fossils of the Jura, it occurred to me to compare those of the upper
and lower slope; and to my surprise I found that they were everywhere
different, and that those of the lower slope were invariably Cretaceous in
character, while those of the upper slope were Jurassic. In the course of
this investigation I discovered three periods in the Cretaceous and four
in the Jurassic epoch, all characterized by different fossils. This led to
a more thorough investigation of the different 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 different organic remains. The attention of geologists
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 scientific men, but by all
who are interested in the mode of origin of animals. Certainly, in the
inland seas of the Cretaceous and subsequent Tertiary 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 periods included within these epochs, with the
organic remains contained in each,--there, if anywhere, 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 deposits, of long intervals, the record of
which has vanished, and which may contain those in