128 RIVERS SHIFTING THEIR CHANNELS. CHAP. VTII. 



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

 an addition to the alluvial plain, and to be only occasionally 

 inundated. In this way, after much encroachment on cliff or 

 meadow in one direction, we find at the end of centuries 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 channels, a flood some 

 times cuts into and partially removes portions 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 afford such reiterated illustrations, and 

 of which a parallel is furnished by the ancient alluvium of the 

 Thames valley, where similar bones of extinct mammalia and 

 shells, including Cyrena fluminalis, are found. 



Professor Noeggerath, of Bonn, informs me that, about the 

 year 1845, when the bed of the Khine 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 Roman 

 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 Rhine to deviate 

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

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

 tools of the stone and iron periods w6uld be found in gravel 

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

 loam deposited above them. 



