In a recent paper simple projective methods were given for determining the displacement of a stratum at a fault.2 It was pointed out that any displacement of a stratum could be represented by a linear displacement and a simple rotation. The rotation was determined by rotating the stratum first around a horizontal axis until it was horizontal, and then around a vertical axis until a line on it was properly oriented; these two rotations were then combined by the ordinary method of combining small rotations, namely, by representing the rotations by lines drawn in the directions of their axes, and with lengths proportional to the amounts of the rotations. The resultant axis is in the direction of the diagonal of the completed parallelogram, and its amount is proportional to the length of this diagonal. The positive direction of the axis is that in which the rotation appears right-handed.
This method, . . .