Page:A Treatise on Geology, volume 1.djvu/56

 40 pushed outwards, so that the strata usually lying uppermost in the group are actually, for a short distance, undermost (e).



Besides these, other forms of disturbed stratification demand attention; especially those in which the continuity of the strata is broken, and the divided parts placed at different levels. This interruption and dislocation of the strata commonly happens along a plane approaching to the vertical, which is usually marked by a rude and irregular fissure. This fissure, whether empty or in any manner filled (with fragments of the bordering rocks or other substances), is called "a fault," and locally "a dyke" "a trouble" "a gall" "a slip," &c.

The most simple and frequent case of faults is represented in the annexed vertical section (No. 9.) at the letter a, the strata lying nearly level, the fault vertical, the dislocation moderate in amount, and no particular bending of the rocks near it. In b the fault deviates from the vertical by the angle x b y, and is said to have an underlay; the strata are considerably depressed, and in such a manner that a perpendicular dropped from b would fall clear of the edges of the depressed beds; not as in c, which represents a rare and exceptional case, so rare, indeed, that a clear example of it with a considerable depression of beds never occurred to the author, among very numerous instances studied in all classes