Valleys of the Bhine and Rhone. 153 



two terraces, in each case, has at one time been filled up at 

 an equal height, so as to form one sheet of alluvial matter, 

 exactly such as a mountain stream may be seen, at this mo- 

 ment, forming in its descent towards a recipient lake, partly 

 above and partly below the water. I therefore conclude that 

 this little river formerly met a recipient body of water much 

 higher than the present lake ; at first, at nearly the height 

 of the upper terrace, afterwards, at the height of the next 

 lower one, and so on ; a cut being always made in the one 

 sheet of alluvial matter on the recipient body falling to the 

 level of the next lower, until the place became a recess 

 winged with terraces, as we now see. 



It was Mr Darwin who first suggested the manner in 

 which ancient deltas were thus intersected, when speculat- 

 ing on such objects, in his well-known paper on Glen Roy. 

 He had observed, that, at the mouths of the little side-val- 

 leys in Glen Roy, there were laid against the hill on each side 

 what he calls " obliquely-truncated buttresses," composed of 

 highly-inclined layers of sand and coarse gravel, the top 

 being, in some instances, on a level with one of the shelves 

 or ancient beaches of the valley. Sometimes a series of 

 these buttresses occurred, one below another. The rivulet 

 of the connected side-valley he shewed to be constantly en- 

 gaged in removing matter from these deposits ; never does 

 it add any. He goes through a process of reasoning, from 

 which he thinks " it becomes evident that the materials of 

 which they are formed were accumulated through the agency 

 of the stream, although it is, at the same time, inconceivablfe 

 that they were left on the steep slope by a force which, as it 

 now acts, is steadily at work, tearing away matter in its 

 whole downward course." He then argues for the necessity 

 of an intervening cause in the action of the streamlet, and, 

 seeing the connection of the flat tops of the principal but- 

 tresses with lines formed by the surface of a body of water 

 occupying the principal valley, demonstrates that that sheet 

 of water must have stood at as many levels as there are but- 

 tresses, receiving the matter of these from the side stream- 

 lets in the form and character of a delta ; the whole being 

 "unequivocal evidence of the check which matter drifted by a 



