G. H. Kinahan — On Denudation. Ill 



the blocks in cliffs and steep mountain sides, from whicli they topple 

 down on to the glaciers, afterwards to be swallowed up in crevasses, 

 and act as tools to triturate, plane, and carve the sides and bottom of 

 the valleys — they also materially assist the marine denudation, loosen- 

 ing the cliffs and land, preparing them for carriage away by the sea. 

 Nevertheless, here the sea, in some way or another, co-operates with 

 the chemical action, for many, if not all rocks at the sea weather 

 more freely than elsewhere/ 



The chemical action is not merely a surface -worker, for its influ- 

 ence can be found at a considerable distance below the surface ; but 

 some " Subeerialists " would wish it to be believed that the chemico - 

 fluvial agencies can do nearly all the work of denudation, while 

 others would give them much greater power than they possess. It 

 has been stated that the parallel valley terraces, such as those 

 described by Dr. Hooker in the valleys of the Himalayas, are due 

 solely to chemico-fluvial agencies, they having been cut in alluvium, 

 which accumulated in flats behind barriers across valleys ; and as 

 the barriers were cut down by the fluvial action, the terraces were 

 formed. This theory at first seems very plausible, but on considera- 

 tion it will be asked : What formed the barrier ? and is the drift 

 behind it always alluvial ? To the first it may be answered, " Cer- 

 tainly not chemico-fluvial agencies, as they have no power to scoop 

 out rocks behind a barrier;" and to the second, "Certainly not," 

 Marine action might possibly have formed them, as we know that the 

 sea, going in or out of ^ long bay or fiord, leaves bars in the narrows, 

 while it cuts deeps above them ; however, as those barriers now 

 under consideration are in mountain valleys and in the vicinity of 

 glaciers, and as it is known that ice can scoop out rocks — Is it 

 not more natural to suppose that these barriers are the result of 

 glacial work ? The chemico-fluvial agents, if they are given time 

 enough, can cut through any barrier, but this will be a work of time 

 and most gradually ; and a gradual lowering of the water above the 

 barrier will not form a cliff but a slope. To form a series of alluvial 

 flats, and to cut in them a series of steps, necessitates a sequence of 

 periods of rest, while the flats were being formed, and of denudation 

 for the steps to be cut, which is quite opposed to an}^ theory of gradual 

 chemico-fluvial denudation. In the Himalayas, however, the forma- 

 tion of the terraces is easily explained, for Dr. Hooker mentions the 

 torrents or debacles of mud and boulders that suddenl}^ come down 

 the slopes of the hills and " dam up more than half a mile of the 

 course of the river into a deep lake." A series of these debacles 

 will account for any number of parallel sets of terraces. It may 

 now be stated that still they are due to chemico-fluvial agencies, as 

 the mud torrents were caused by those powers. This is unquestion- 

 able ; but at the same time it may be pointed out that they had as an 

 auxiliary, glacial action : for it was the breaking up of the glaciers 

 which caused the debacles.- 



^ See short notes on this subject by the writer, Geol. Mag. Vol. Y. p. 18. 

 '^ Capt. Forbes, R.N., in " Iceland, ItsVolcanos, etc.," pp. 281, et sequit. mentions 

 huge debacles caused by glaciers being loosened and displaced by Yolcanic heat. 



