Page:A Treatise on Geology, volume 1.djvu/67

 CHAP. III.

Whenever, in any district, the stratified rocks, instead of all lying parallel to one another, suffering the same deviations from horizontality, bending in the same flexures, dropping or rising by the same faults, and regarding in their declinations the same axis or centre of dips, divide themselves, in these respects, into two or more sets, which differ from one another in all or any of these respects, there is said to be unconformity of the strata. The place of this unconformity is the interval between the two sets thus disagreeing; it is said to occur between the oldest of one and the newest of the other set; it affects the geographical distribution of the strata, as shown on a map, and their relative inclination and exposures, as shown in a section. Thus in the map diagram (fig. 15.),



the series of strata marked a b c d are parallel in one set, and e f g in another; but their directions, or strikes, (S, S and s s) on the surface differ, and the lowest of the upper set (e) rests in one place on a, in another on b; in another on c or d.; the dips, D and d, are in different directions.



In a section (as fig. 16.), some difference of inclination