Correspondence — Rev. T. G. Bonney. 427 



above the present level of the existing streams ; (2) the general paral- 

 lelism of the scars on opposite sides of the valleys ; (3) the persist- 

 ence of the peculiarities in form due to the nature of the bed. But 

 with regard to these objections, I may remark that, so far as my 

 memory serves me, they would hold -in every hilly district that I 

 have seen, where bedded rocks of a similar character exist, whether 

 the district has been exposed to glacial action or not ; also that it is 

 no uncommon thing to see a river valley, with nearly flat bed, far 

 wider than the existing stream. In some cases these may be 

 explained by the volume of the stream being formerly greater, as it 

 no doubt was occasionally in past history ; in others the slow motion 

 of the river from one side of the valley to the other would suffice. 

 Further, that if a configuration were once given to the banks of the 

 valley, and these were afterwards cut back in tolerably homogeneous 

 rock by aerial denudation only, the original form would still be gen- 

 erally preserved, because the recession would be approximately 

 uniform throughout. These phenomena are to be seen in the 

 valleys of the Alps and the Jura, and if he is prepared to attribute 

 these mainly to glacial erosion, he must get over some objections to 

 which I will presently refer. 



Is it necessary that swallow-holes should be formed by streams ? 

 Of course streams may form them — witness Gaping Gill ; but I have 

 seen districts riddled by swallow-holes, as the " Stony Seas " of the 

 Eastern Alps, which were evidently formed by the rain drainage of 

 a very small area. Some of those also in the Chalk have, I think, 

 been simply dissolved out by the subterranean drainage of a very 

 limited basin. • The formation of a swallow-hole, I think, mainly 

 depends on the nature of the rock. I have, however, seen in the 

 Alps swallow-holes which have been modified by the erosive action 

 of the streamlet. 



The large amount of debris supposed to have existed on the surface 

 before the Glacial Period seems to me an assumption. Many parts 

 of the great insular mass of crystalline rock in Central France had 

 almost certainly not been under water from a very remote epoch, a 

 large portion of it certainly not since Miocene times ; yet there is no 

 great amount of surface debris here, and I suppose we may dispense 

 with an ice-sheet for Auvergne ? Surely also, as soon as a layer of 

 a few feet of debris had formed on the surface, it would greatly pro- 

 tect the rock beneath from all agencies but those of percolating water ? 

 Again, with regard to Mr. Goodchild's explanation of the absence of 

 ice-marks from the higher parts of mountains (p. 360). If a glacier 

 is an erosive agent of such power, as he supposes it to be, a very 

 limited duration of contact with the upper rocks ought to suffice for 

 imprinting its "handwriting on the wall." Thus, making every 

 allowance for greater exposure to weather, we ought now and then 

 to find the blurred remnants of these inscriptions. I have climbed 

 more than most men in the regions of glaciers, but never saw them. 

 To my eyes the transition from weather-worn to ice-worn rocks 

 appears usually rather abrupt, and at a regular height. 



Finally is there evidence at all that glaciers possess this immense 



