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 surface of the earth, accompanied by the introduction of a new set of animals, though the changes in the organic world are not so striking as those which coincide with the mountain-upheavals.  Still, to the eye of the geologist they are quite as distinct, though less evident to the ordinary observer.  To these divisions it seems to me that the name of Period is rightly applied, because they seem to have been brought about by the steady action of time, and by gradual changes, rather than by any sudden or violent convulsion.

It was my good fortune to be in some degree connected with the investigations respecting the limitation of Periods, for which the geology of Switzerland afforded peculiar facilities.  My early home was near the foot of the Jura, where I constantly faced its rounded domes, and the slope by which they gently descend to the plain of Switzerland.  I have heard it said that there is something monotonous in the continuous undulations of this range, so different from the opposite