Unconformability of Strata.— Strata are said to be unconformable when one series is so placed over another that the planes of the superior repose on the edges of the inferior (see Fig. 78). In this case it is evident that a period had elapsed between the production of the two sets of strata, and that, during this interval, the older series had been tilted and disturbed. Afterwards the upper series was thrown down in horizontal strata upon it. If these superior beds, _d, d,_ Fig. 78, are also inclined, it is plain that the lower strata _a, a,_ have been twice displaced; first, before the deposition of the newer beds, _d, d,_ and a second time when these same strata were upraised out of the sea, and thrown slightly out of the horizontal position. Fig. 79: Junction of unconformable strata near Mons, in Belgium. It often happens that in the interval between the deposition of two sets of unconformable strata, the inferior rock has not only been denuded, but drilled by perforating shells. Thus, for example, at Autreppe and Gusigny, near Mons, beds of an ancient (primary or palæozoic) limestone, highly inclined, and often bent, are covered with horizontal strata of greenish and whitish marls of the Cretaceous formation. The lowest, and therefore the oldest, bed of the horizontal series is usually the sand and conglomerate, _a_, in which are rounded fragments of stone, from an inch to two feet in diameter. These fragments have often adhering shells attached to them, and have been bored by perforating mollusca. The solid surface of the inferior limestone has also been bored, so as to exhibit cylindrical and pear-shaped cavities, as at _c_, the work of saxicavous mollusca; and many rents, as at _b_, which descend several feet or yards into the limestone, have been filled with sand and shells, similar to those in the stratum _a._ Overlapping Strata.—Strata are said to overlap when an upper bed extends beyond the limits of a lower one. This may be produced in various ways; as, for example, when alterations of physical geography cause the arms of a river or channels of discharge to vary, so that sediment brought down is deposited over a wider area than before, or when the sea-bottom has been raised up and again depressed without disturbing the horizontal position of the strata. In this case the newer strata may rest for the most part conformably on the older, but, extending farther, pass over their edges. Every intermediate state between unconformable and over-lapping beds may occur, because there may be every gradation between a slight derangement of position, and a considerable disturbance and denudation of the older formation before the newer beds come on. [1] See “Principles of Geology,” 1867, p. 314. [2] Edin. Trans., vol. vii, pl. 3. [3] Proceedings of Geol. Soc., vol. iii, p. 148. [4] Thurmann, “Essai sur les Soulèvemens Jurassiques de Porrentruy,” Paris, 1832.