determined by some
decisive event. In speaking of human affairs, we say, "It was an epoch or
an era in history,"--or in a more limited sense, "It was an epoch in the
life of such or such a man." It at once conveys the idea of an important
change connected with or brought about by some striking occurrence. Such
were those divisions in the history of the earth when a violent convulsion
in the surface of the globe and a change in its inhabitants ushered in a
new aspect of things.

I have said that we owe to Elie de Beaumont the discovery of this
connection between the successive upheavals and the different sets of
animals and plants which have followed each other on the globe. We have
seen in the preceding article upon the formation of mountains, that the
dislocations thus produced show the interruptions between successive
deposits: as, for instance, where certain strata are raised upon the sides
of a mountain, while other strata rest _unconformably_, as it is called,
above them at its base,--this term, unconformable, signifying merely that
the two sets of strata are placed at an entirely different angle, and must
therefore belong to two distinct sets of deposits. But there are two
series of geological facts connected with this result which are often
confounded, though they arise from very different causes. One is that
described above, in which a certain series of beds having been raised out
of their natural horizontal position, another series has been deposited
upon them, thus resting unconformably above. The other is where, one set
of beds having been deposited over any given region, at a later time, in
consequence of a recession of the sea-shore, for instance, or of some
other gradual disturbance of the surface, the next set of beds accumulated
above them cover a somewhat different area, and are therefore not
conformable with the first, though parallel with them. This difference,
however slight, is sufficient to show that some shifting of the ground on
which they were accumulated must have taken place between the two series
of deposits.

This distinction must not be confounded with that made by Elie de
Beaumont: we owe it to D'Orbigny, who first pointed out the importance of
distinguishing the dislocations produced by gradual movements of the earth
from those caused by mountain-upheavals. The former are much more numerous
than the latter, and in every epoch geologists have distinguished a number
of such changes in the