It indicates, beyond the possibility of misconception, that there was an interval between the deposition of the older series and that of the newer series of strata; and that during this interval the older beds were raised above the sea-level, so as to form dry land, and were subsequently depressed again beneath the waters, to receive upon their worn and wasted upper surface the sediments of the later group.

During the interval thus indicated, the deposition of rock must of necessity have been proceeding more or less actively in other areas.

Every unconformity, therefore, indicates that at the spot where it occurs, a more or less extensive series of beds must be actually missing; and though we may sometimes be able to point to these missing strata in other areas, there yet remains a number of unconformities for which we cannot at present supply the deficiency even in a partial manner.
[Illustration: Fig.

18 .-- Section showing strata of Tertiary age (a) resting upon a worn and eroded surface of White Chalk (b), the stratification of which is marked by lines of flint.] It follows from the above that the series of stratified deposits is to a greater or less extent irremediably imperfect; and in this imperfection we have one great cause why we can never obtain a perfect series of all the animals and plants that have lived upon the globe.

Wherever one of these great physical gaps occurs, we find, as we might expect, a corresponding break in the series of life-forms.