can be found, and a certain mean angle of dip assigned to the whole mass.
Fig. 31.
If, for instance, Fig. 31, there be a set of beds, including one particular bed A A' dipping at 30° in one direction, traversed by the fault B C running in the direction of the dip, and the ends of the bed A A' on opposite sides of the fault be any distance apart, say 200 yards, then, inasmuch as the bed A would by the table be twice 58=116 yards deep at D on one side of the fault while it is at the surface of the ground (supposed to be a horizontal plane) on the other side of the fault, it is obvious that the fault B C has a "downthrow" of 116 yards towards D.
If the fault traverse the beds obliquely to the strike, as in the following figure—
Fig. 32.
—we must, instead of measuring along the fault B C, of course, measure A D along the dip, and then proceed as before.
Conversely when the amount of the throw of any fault, and the angle of inclination of the beds, are known, if the place of the bed on one side of the fault be ascertained, that of its outcrop on the other side of the fault may be calculated, and so on.
Oblique Sections.
Although it has no especial reference to the district of South Staffordshire, I would yet take this opportunity of printing and publishing the additional table below.
In the year 1850 we were engaged in running sections across a very contorted district of North Wales, so contorted that it was impossible to contrive any long continuous section that should not in some part of its course cross both beds and cleavage planes very obliquely. It became important, therefore, to know what correction to apply to the observed angle of dip of those beds and planes, so that they should be
