"Although all these formations have been deposited by water,
and although they may all be found in the same locality lying
upon each other, the passage from the one to the other is never
made by insensible gradations. A sudden and marked change is
always to be perceived in the physical nature of the deposit,
and in that of the organized beings whose remains are found in
it. Thus it is evident, that between the epoch at which the
limestone of Jura was deposited, and that of the precipitation
of the system of greensand and chalk which covers it, there has
been upon the surface of the globe a complete change in the
state of things. The same may be said of the epoch that
separates the precipitation of the chalk from that of the
tertiary formations; as it is also evident that in every place
the state or nature of the liquid, whence the earths were
precipitated, must have changed completely between the time of
the formation of the tertiary strata, and that of the diluvium.

"These considerable variations, sudden, and not gradual, in the
nature of the successive deposits formed by the waters, are
considered by geologists as the effects of what they call '_The
Revolutions of the Globe_.' And even although it is very
difficult to say exactly in what these revolutions consisted,
their occurrence is not the less certain on that account.

"I have spoken of the chronological order in which these
different sedimentary strata have been deposited: I must
therefore state that this order has been determined by
following, without interruption, each different formation, to
those regions in which it could be ascertained beyond question,
and over a great horizontal space, that some particular layer
was above some other. Natural excavations, such as the cliffs
that border the sea, common wells, and Artesian fountains, with
the excavation of canals, have furnished powerful aid in this
inquiry.

"I have already remarked, that all these sedimentary formations
are stratified. In level countries, as might be expected, the
disposition of the layers is nearly horizontal. In approaching
mountainous countries, this horizontality, generally speaking,
ceases; finally, on the sides of mountains, some of these
layers are very much inclined; they even sometimes atta