present external configuration. In nature, while the lateral pressure and ridging up were in process, sub-aërial denudation at a rate somewhat less rapid would have been cutting down and truncating the folds and ridges of our mountain range, and so exhibiting to us the wonderful structures the genesis of which we are investigating.

L

CHAPTER XI

EXPERIMENTAL MODELS

N 'The Origin of Mountain Ranges,' pp. 331-33,

I gave a description of certain experiments made by me in illustration of some of the principles expounded in that work. In consequence of these having been undertaken while the book was in the press the matter was necessarily compressed. This was in the year 1886. Since then more elaborate experiments have been made by Cadell on the same lines,' and later still by Bailey Willis.2 Their apparatus and methods of compression were in principle the same as those employed by me and described in the pages referred to.

The objects aimed at were, however, somewhat different. Cadell was desirous of discovering in what way the huge thrust planes and intense folding of the Highland rocks were produced. Bailey Willis held the same object in view with regard to the Appalachians, but also aimed at elucidating general principles. My investigations

1 Experimental Researches in Mountain Building' (Trans. Royal Soc. of Edin., vol. xxxv. Part 7, 1888).

2 The Mechanics of Appalachian of Appalachian Structure' (Thirteenth Annual Report of the U. S. Geological Survey, 1891-92; published 1894).

were undertaken partly to test some of the principles previously geometrically reasoned out in the body of the work.

I have thought it worth while in this further consideration of the subject to give photographs of some of the results arrived at in 1886. Being of clay, the contorted beds in the models have naturally shrunk after fifteen years' keeping in a dry cupboard, but not so as to vitiate the original description given in 'The Origin of Mountain Ranges.'

In addition, I give diagrams and photographs of experiments on composite bars made up of sheetlead and paper and sheet-lead, millboard and paper. I also give some curious results of the compression of moist sand. These experiments were all made between December 1886 and February 1887.

I must here take the opportunity of remarking that, from the complexity of the subject, it is necessary in such experimental investigations to deal with one phase at a time of the movements the earth's crust is subject to.

Compression of Composite Bars.-The experiments detailed by Cadell and Willis were made upon composite bars of clay, plaster, and other materials of varying tenacity and plasticity, grouped in a trough and compressed from the ends. This produces vertical folds, accompanied with compression on the concave and tension on the convex side, the movements being in parallel vertical planes. To any one who has studied The Origin of

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Mountain Ranges' it is hardly necessary to say that this is what seldom occurs in nature. Anticlinals, as there pointed out, are mostly ellipsoidal in shape, and are due to converging compressive forces; that is, to centripetal pressure.

Domical Structures.-In the experiment shown in fig. A, Plate XLII., 'Origin of Mountain Ranges,' I attempted to produce this effect of converging pressures, with partial success.

I now give photographs of the resultant model, which may be called a truncated anticlinal (Plate VII.). This shows its gradual development from an anticlinal of a small sectional area in the bottom bed to a larger one in the middle, increasing to a maximum at the top. It will be observed that the anticlinal is curved not only transversely, but also at right angles thereto, being a truncated ellipsoid, due to the pressure producing it having been to a small extent converging. This experiment is described in detail on page 163, Experiment No. 7.

Differential Expansion, the Cause of Folding and Domed Structures.-But it is by the expansion of lead plates by differential heating that the most instructive results have been obtained.

Such differential expansion produces a much more exact imitation of the forces producing the folding and compression that takes place in the building up of a mountain range. The forces are in the case of differentially heated plates internal to the affected area, whereas in the experiments by the authors named, and by others who have