348 LORD AVEBURY ON AN [Aug. I903, 



26. An Experiment in Mountain-Building. By the Right Hon. 

 the Lord Avebury, P.C., D.C.L., LL.D., F.R.S., F.G.S. (Read 

 May 27th, 1903.) 



Many years ago Sir James Hall illustrated the formation of folded 

 mountains by placing layers of cloth under a weight, and then 

 compressing two of the sides so that the cloth was thrown into folds. 

 Since then, other and more complete experiments of the same kind 

 have been made by Favre, Cadell, Daubree, Willis, and Ruskin. 



In these experiments the compression was from two sides. If, 

 however, folded mountains are caused by compression due to the 

 contraction of the earth, the compression must take place in two 

 directions at right angles one to the other. 



With the view of illustrating this I consulted Mr. Horace Darwin, 

 and he constructed for me an apparatus consisting of four square 

 beams of wood, resting on a floor, which by means of screws could 

 be moved nearer to, or farther from, each other. The beams left 

 between them a space 2 feet across and 9 inches in depth. 



In the square central space I placed some pieces of carpet-baize 

 and hryers of sand, each about 1^ inches deep. About an inch 

 above the upper layer of sand I placed a piece of plate-glass and 

 some weights. The machine was then set in motion, causing the 

 beams of wood to approach one another. The sand rose in the 

 centre, until it reached the glass, when it was flattened out. 



On removing the upper layer of sand, the top-piece of cloth was 

 as shown in fig. 1 (p. 350), which is a photographic reproduction of 

 a cast in plaster-of-Paris. The upper surface is gently undulating 

 in the centre, with some steep folds near the edges and one slight 

 ridge crossing the plateau at right angles to one of the folds. 



On removing the underlying layers of sand, the next layer of cloth 

 was as shown in fig. 2 (p. 350) . There are two main lines of elevation ; 

 one running from each corner, and consequently crossing at right 

 angles. The whole surface forms a series of winding and curving 

 ridges with intervening valleys, and gradually rises to a culminating 

 dome a little on one side of the centre, where the two main ridges 

 intersect. I was rather surprised at the marked difference between 

 this and the upper layer. 



Underneath this second piece of baize was another layer of sand, 

 on the removal of which the third layer of baize was found to be 

 thrown into folds as in fig. 3 (p. 351). This again differed greatly 

 from, though it evidently followed the same general law as, the pre- 

 ceding. The ridges are narrower and more pronounced, the valleys 

 more precipitous. There is also a marked tendency for each ridge 

 to present a central longitudinal division. 



Fig. 4 (p. 351) represents a fourth layer of baize, which was separated 

 from the third by about 1| inches of sand, and from the bottom of 

 the apparatus by a similar layer. This fourth layer of baize differs 

 from the third in somewhat the same manner as the third differs 



