H4 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 5, 



of their origin is, I believe, universally admitted : but it has been so 

 clearly stated by Mr. Darwin, that I will quote his words ; in case of 

 " a river delivering during a long period detritus into a lake, the 

 level of which was gradually sinking from the wearing down of its 

 mouth, a gently sloping surface would be formed at its bead. But 

 as the barrier was cut deeper and deeper, and the lake sank, the 

 stream in the part where it was once checked by meeting with the 

 still water would gain velocity, and hence would cut through the 

 beds which it had originally deposited; ''"' and again, at p. 55, — 

 *' the waterworn materials appear to have been transported by the 

 present rivers, and yet they are so deposited as could not have hap- 

 pened without some intervening cause. The only obvious solution 

 is that the valley had been occupied by an expanse of gradually sub- 

 siding water, either of a lake or of an arm of the sea." 



Besides the matter brought down by the rivers, the terraces in the 

 Swiss valleys have in many instances received large additions of 

 materials from the falling of fragments of rock down the steep sides 

 of the mountains ; and the outlines of these accumulations give a 

 strong confirmation to the above views ; under ordinary circumstances 

 the fragments which roll down the sides of a precipice form them- 

 selves, as is well known, into a slope at an angle of about 35°, which 

 has been named in consequence mg. S.-Diagram to show the arrauffe- 

 "the angle ot repose : but m menf of the materials in the Terraces. 

 the lower parts of the Swiss val- 

 leys it is very common to observe yy; 

 that the slope suddenly changes /^'''-'' 

 from 35° in the upper part of the ^yyy'y'' 

 talus to about 15° in the lower yyyyy'' 

 part, as at a in the annexed dia- ^^--^^ V'''''^ 

 gram (fig. 3) ; and this change ^^.,J^h:^^^<^<-'^^^^^^^^ 

 of angle takes place at a uniform ■<^<!^-::'----^^^^^^ 

 height for a considerable distance. 



This is well seen on the south side of the valley of the Rhone near 

 Sierre, where the level at a is 1800 or 2000 feet above the sea, 

 and the lower slope spreads out into a series of flat hillocks more 

 than 200 feet above the Rhone. The change of angle of the slope 

 of the debris, and the uniformity of level at which the change takes 

 place, are strong proofs that water stood at this time up to the level 

 at a, and that this water had sufficient motion, either from currents 

 or tides, to alter the angle of subsidence of the falling debris f . 



The diagram, fig. 4, gives the outline of two terraces in the Val- 

 telline, above Grosio ; the angles are drawn correctly, but the distance 

 between the two ends of the upper terrace at Sandalo and Maggione 



* Darwin, " On the Parallel Roads of Glen Roy : " Phil. Trans. 1839, p. 51. 



t In Nov. 1830 Mr. James Yates communicated a paper to this Society " On 

 the Formation of Alluvial Deposits," of which a short abstract is given in our 

 Proceedings, vol. i. p. 237 : it will be found at length in the Edinburgh New Phil. 

 Journ. for 1831, vol. xi. p. 1. The manner in which terraces are formed in lakes 

 from the materials brought down by the streams is well explained, pp. 30-39. 

 See also R. Chambers, * Ancient Sea Margins,' p. 51. 



