Fig. 82.
Curved strata of the Iselten Alp.
Fig. 83.
Unconformable junction of old red sandstone and Silurian schist at the Siccar Point, near St. Abb's Head, Berwickshire. See also [Frontispiece.]
Unconformable stratification.—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. 83.]). 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, as d, d, [fig. 83.], 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 thrown out of the horizontal position.