Plate X., Fig. 1.-This represents an octagon formed out of similar paper to fig. 1, Plate IX. It is 6 inches across and scored with a knife alternately on opposite sides, but in this instance. with radial curves. The area by measurement is 29-73 square inches. Fig. 2 represents the same sheet in the folded form, the basal area being 10.98 square inches, or 18.79 less than the original octagon. The folded surface area is precisely the same as fig. 1, and is shown by the enclosing white line.

The interesting feature of this experiment is that in the folding the central area takes on a spiral movement. To show this effectively a pointer has been pinned to the central part. On comparison of this with the fixed pointer, it will be seen that it has moved round through 26°.

It must not be thought that Nature ever exactly reproduces the conditions of any of the experiments by which we have been trying to diagnose earth movements and get a grip of the conditioning factors.

The last experiment bears upon the possibility of strata in compression developing torsion structure such as Mrs. Ogilvie Gordon has investigated in the Dolomites.1

1

In what Professor Lapworth aptly calls tri

Q. J. G. S., vol. lv. pp. 560-634. See also report of meeting of the Geo. Soc. of Edinburgh (Nature, February 12, 1903), where Dr. Gordon gave her views of Simultaneous Duplex Crust Movements in the Fassa-Monzoni District.'

dimensional movements it seems as if anything were possible, even 'mushroom-shaped mountains. without roots.'

Further experiments will be detailed bearing upon this interesting problem.

CHAPTER XIII

DETAILS OF EXPERIMENTAL INVESTIGATIONS

THE FOLDING OF COMPOUND BARS SUBJECT TO END

EXPERI

COMPRESSION

XPERIMENT No. 1.—This was made with a series consisting of eight plates of lead, weighing 7 lb. to the superficial foot, measuring 1 ft. 83 in. long by 2 in. wide. Between each pair two layers of calico were placed. The combination A, fig. 1, Plate XI., was then placed on a 'sole' board, a, 2 ft. 5 in. long, having a side secured thereto for guiding the combination. Two pieces of board, b, b, were placed at each end, on top of the lead series, with a bridging board, c, covering all and spanning the space between. These arrangements will be easily understood on referring to Plate XI. The combined series was secured together and to the 'sole-board' by clamps at d, d.

The object of the space e was to allow the combined bar to rise in an anticlinal at that portion of the bar.

By means of a powerful cramp pressure was applied to the ends of the lead combination bar, which slid between the cheeks by which it was

confined. At first it took a considerable force to turn the screw of the cramp, and the lower plates at f, fig. 2, became contorted.

Continuing to turn the screw, the crown of the anticlinal (h, fig. 2) rose to 25 in. above the soleboard at g, the lead bar being shortened to 1 ft. 8 in. 5 When shortened to 1 ft. 7 in. the height was 3 in.; when 1 ft. 7 in. the height was 33 in., and, when 1 ft. 7 in., 4 in., as shown in fig. 3.

The rise of the crown of the anticlinal, which brought it in contact with the bridging board, so bent the latter upwards that at this point it cracked at i, fig. 3.

The bridging board was now removed, and the screwing up continued until an overturn-anticlinal was formed, as shown in fig. 4, which measured 8 in. high and 93 in. at base, or a reduction of 11 in. in length of the base.

The pressure of the bridging board did not seem to appreciably influence the form of the anticlinal, and serves to show how great must be the lifting effect in nature of a rising and developing anticlinal upon the covering rocks.

The photograph, Plate XII., fig. 1, shows the completed overturn-anticlinal in perspective, from which a better idea of it as a solid can be obtained than from the diagrams.

NOTE. I have not thought it necessary in figs. 2, 3, and 4 to draw in the individual plates, but have treated the combination as a bar. The photo