be a weak line in the strata and a large area of expansion compared with the thickness of the strata. Explanation of Plate XX.-Fig. 1. Finished model as taken out of the compressor, showing truncation of linear fold against the zinc band. Fig. 2. Model with top stratum removed. Experiment No. 17 (Plate XXI.): Compression of Clay Plate moulded over a Helical Core.-This may be designated a 'sporting' experiment. A helical core was laid down on the metal bedplate, and a disc of very plastic clay, ğ inch thick, laid upon and moulded to it (fig. 6, Plate XVII.). This homogeneous plate behaved in a singular and unexpected manner. On being compressed to the fullest extent the machine was capable of, it became in general form like a ram's horn, measuring 71⁄2 inches on the longer diameter and 65 inches on the shorter (see fig. 1, Plate XXI.). The periphery thickened from 1 to 2 inches, making the underside of the model cup-shaped, averaging about 1 inch deep. The coil or helix became rounded in section, and a similar coil was developed on the underside, apparently independently of the top one. In cross-section the two helices are anticlinals, and the hollow between a 'synclinal' (see fig. 2, Plate XXI.). Lines were drawn at right angles across the 1 When the model is reversed the two anticlinals become synclinals and the synclinal an anticlinal. circle before compression, which after compression showed a spiral movement from left to right, or in the opposite direction to that of preceding experiment. This model was the result of a series of complex movements very difficult to analyse. It appeared. that the helical form gave it great strength to resist the circumferential pressure, which turned down and thickened the periphery of the plate, at the same time developing the helical folds and producing the cup form of the underside. Although not simulating any known earth structure, it shows what infinite possibilities of change of form by compression may exist in the earth's strata. Explanation of Plate XXI.-Fig. 1 is a view of the model as it came from the compressor, with the ammonite-looking coil developed on top surface. It will be observed that the bottom rim is irregular on the edge, which is pressed up in places in a ragged fringe. Fig. 2. View of the underside. The internal fold is part of the helix which was developed on the underside of the model. It is the convex side of the synclinal. This view shows well the turning down and thickening of the peripheral rim. The rounded form of the cross-section of the helix is evidently produced by folding. To describe the model in a phrase, it may be designated a helical folding.' Experiment No. 18 (Plate XXII.): Spiral Fold |