The main difference is, that instead of being acted upon by equal multilateral or circular compression, the forces that upheaved the Wrekin were dominant along the shorter axis.

Explanation of Plate XVIII.-Fig. 1 shows the whole compound structure complete, as taken out of the compressor.

Fig. 2 shows the three top layers removed, the lower layer remaining on the base bed.

Fig. 3 shows the whole of the layers or concentric domes removed, and exposes the irregular surface of the base bed, with its oblique bedding having a high dip.

The dark surfaces are due to the black sand, as already referred to.

Experiment No. 15 (Plate XIX.): Torsion Structure. This experiment was conducted with the object of testing the possibility of torsion structure arising from the compression of a combination of solid plates.

It was discovered by the experiment detailed in p. 138 that a flexible developable surface of paper of octagon form, as illustrated in the model Plate X., on being compressed at the edges, developed a spiral movement in the centre, made manifest by the movement of the attached index finger.

In the circumferential compression of clay discs about to be described it was found in some cases advisable to initiate a desired movement by giving

anticlinal. As to whether the core is formed by faulting or moulding' will depend largely upon its rigidity.

the structure what I term, for want of a better name, an initial bias.'

In this case (Experiment No. 15) the initial bias was obtained by placing strips of clay of a triangular section upon the oiled metal bed-plate. These radiated in curved lines from a fixed centre, 'a,' which was eccentric to the true centre of the circle (see Plate XVII., fig. 4).

Upon this bed-plate with curved radial ridges, b, b, oiled on upper surface, was placed a sheet of clay, of an inch thick, carefully moulded to the underlying ridges. This sheet was dusted over with very fine sand' and two other sheets superposed.

The assemblage was placed in the compressor, and was gradually screwed up until the base became 7 inches on the longer and 7 inches on the shorter diameter.

The upper part of the structure, not being so much compressed, was somewhat larger than the base.

To determine the torsional movement (if any) that might take place, straight lines were pricked across the surface of the model.

Folds arose on the periphery at four points, the terminations of the radial ridges, b, b, b, b. These folds had overthrows to the left, and the finished model (Plate XIX., fig. 1) indicates a general spiral

1 The sand used throughout was obtained from the washings and siftings of glacial clays of various degrees of fineness. The surfaces to be photographed were also dusted over, as it was found the pictures came out better when so taken.