Page:A Treatise on Geology, volume 1.djvu/66

 50 angle below, and covered up by horizontal strata of red marl and lias, thus:

Lias. Red marl. Coal formation.

In Yorkshire and Durham the same thing is observed with respect to the magnesian limestone and the coal, with the addition that the coal strata are broken by faults, which do not affect the limestone above; thus:

The principal difficulty in applying this very simple mode of determination to particular cases, so as to class all faults and other effects of subterranean movements according to the date of their occurrence, consists in ascertaining the indispensable data of what strata are and what are not disturbed along a given line, or at a certain point. When the undisturbed strata lie upon, or abut clearly against, or plainly surround the disturbed rocks, the evidence is satisfactory, and easily verified; but, in most cases, this clear testimony is wanting, and it is by considering the relative directions and relative dips of the two sets of strata (the disturbed and undisturbed), that we are to arrive at a determination of the question. The following notices and sketches will illustrate this point.