128 RIVERS SHIFTING THEIR CHANNELS. chap. vni. 



shallower, and is soon destined to be raised so high as to 

 form an addition to the alhivial plain, and to be only occa- 

 sionally inundated. In this way, after much encroachment 

 on cliff or meadow in one direction, Ave find at the end of cen- 

 tui'ies that the width of the channel has not been enlarged, 

 for the new-made ground is raised after a time to the full 

 height of the older alluvial tract. Sometimes an island is 

 formed in mid-stream, the current flowing for a while on 

 both sides of it, and at length scooping out a deeper channel 

 on one side so as to leave the other to be gradually filled up 

 during freshets and afterwards elevated by inundation-mud, 

 or "brick-earth." During the levelling up of these old chan- 

 nels, a flood sometimes cuts into and partially removes jjortions 

 of the previously stratified matter, causing those repeated 

 signs of furrowing and filling up of cavities, those memorials 

 of doing and undoing, of which the tool-bearing sands and 

 gravels of Abbeville and Amiens aftbrd such reiterated illus- 

 trations, and of which a parallel is furnished by the ancient 

 alluvium of the Thames valley, where similar bones of extinct 

 mammalia and shells, including Cyrcna fiuminalis, are found. 



Professor Noeggerath, of Bonn, informs me that, about 

 the year 1845, when the bed of the Ehine was deepened artifi- 

 cially by the blasting and removal of rock in the narrows at 

 Bingerloch, not far from Bingen, several flint hatchets and 

 an extraordinary number of iron weapons of the Eoman 

 period were brought up by the dredge from the bed of the 

 great river. The decomposition of the iron had caused much 

 of the gravel to be cemented together into a conglomerate. 

 In such a case we have only to suppose the Ehine to deviate 

 slightly from its course, changing its position, as it has often 

 done ill various parts of its plain in historical times, and then 

 tools of the stone and iron periods would be found in gravel 

 at the bottom, with a great thickness of sand and overlying 

 loam deposited above them. 



