A proper interpretation of some geological phenomena requires that allowance shall be made for differential earth-movements that have taken place since the period of occurrence of the events or conditions under consideration.
Present differences of level in rocks of the same age may be partly due to actual differences in depth of the sea-floor on which they were deposited, but they may also be the results of subsequent differential earth-movements either of a regional or of a local character, the latter including ‘fauhs.’
In order to estimate the amount of displacement or differential movement, it is necessary to select a particular rock as a datum. The rock selected should combine, as far as possible, the following characteristics:—
1. It should be comparatively thin.
2. It should have a considerable horizontal extension.
3. It should combine similarity in physical characters and palæontological contents over a large area, so that uniformity of depth of deposit may be postulated.