This is the true dip. But the edges of highly
inclined strata may give rise to perfectly horizontal lines in the face of a
vertical cliff, if the observer see the strata in the line of the strike, the
dip being inward from the face of the cliff. If, however, we come to a break in
the cliff, which exhibits a section exactly at right angles to the line of the
strike, we are then able to ascertain the true dip. In the drawing (Figure 61),
we may suppose a headland, one side of which faces to the north, where the beds
would appear perfectly horizontal to a person in the boat; while in the other
side facing the west, the true dip would be seen by the person on shore to be at
an angle of 40 degrees. If, therefore, our observations are confined to a
vertical precipice facing in one direction, we must endeavour to find a ledge or
portion of the plane of one of the beds projecting beyond the others, in order
to ascertain the true dip.
(FIGURE 62. Two hands used to determine the inclination of strata.)
If not provided with a clinometer, a most useful instrument, when it is of
consequence to determine with precision the inclination of the strata, the
observer may measure the angle within a few degrees by standing exactly opposite
to a cliff where the true dip is exhibited, holding the hands immediately before
the eyes, and placing the fingers of one in a perpendicular, and of the other in
a horizontal position, as in Figure 62.
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