Explanation of Plates XXVI. and XXVII.— Plate XXVI. View showing the moving thin edge of the overshear, which is beautifully slickensided on the underside. The clay plate where not sheared now measures inch thick-that is, it has

undergone a thickening of inch.

Plate XXVII. View looking towards the point of the overshear, showing the unsheared clay plate forming the base and the portion of the overriding clay, or overshear,' where it has left the undershear and been carried forward to the right by the peripheral movement.

Experiment No. 23 (Plate XXVII.): Spiral Shearing (continued).-A homogeneous plate of clay, somewhat stiffer than in Experiment No. 22 and 14 inch thick, was prepared and put into the compressing-band.

On screwing up the clay began to thicken and shear from the periphery to the centre, opposite the tightening-screw, as shown at c in diagram, fig. 7, Plate XXIV. This developed into a convex rise as shown at d, fig. 8. At the same time a lunette anticlinal began to show itself at e, fig. 8, following the periphery, and the overshear began to lift. The spiral movement was in this case to the left, or in the opposite direction to that of the preceding experiment.

The third phase, as shown in diagram, fig. 9, was an intensified form of that exhibited in diagram, fig. 8.

Finally the shear-plane became well developed,

as shown in Plates XXVIII. and XXIX., the point of the spiral rising 54 inches above the base. The overshear and the undershear were well slickensided. The shear was more abrupt and not so regular as Experiment No. 22.

The underside of the compressed disc of clay was found to be a volute-formed hollow, the result of combined compression and torsional movement, the complement of the convexly screwed upper surface.

The finished base (fig. 10, Plate XXIV.) measured when dry 7 by 63 inches.

Explanation of Plates XXVIII, and XXIX.Plate XXVIII. A A photographic view of the finished model as taken from the compressor, showing the surface form assumed by the disc and the upper surface form and rise of the overshear, together with the numerous minute screwshears which developed on the surface of the clay.

Plate XXIX. View of the same model taken from a point opposite the shear-plane.

The original plate of clay thickened about 4 in., one of the results of compression.

Experiment No. 24 (Plate XXX.): Spiral Domical Overfold. Three plates of clay, averaging together & inch thick, or inch each, were laid upon three small lenticular cores arranged in the form of a triangle on the metal bed-plate; but these cores did not seem to influence the movements.

On screwing up a fold began to develop near