m

RIVERS SHIFTING THEIR CHANNELS.

CHAP. IX.

the older alluvial tract. Sometimes an island is formed inmidstream, the current flowing for a while on both sides ofit, and at length scooping out a deeper channel on one side soas to leave the other to be gradually filled up during freshetsand 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 previouslystratified matter, causing those repeated signs of furrowingand filling up of eavities, those memorials of doing andundoing, of which the tool-bearing sands and gravels ofAbbeville and Amiens afford such reiterated illustrations, andof which a parallel is furnished by the ancient alluvium ofthe Thames valley, where similar bones of extinct mammalia,and shells including Cyrena fluminalis, are found.

Professor Noeggerath, of Bonn , informs me that, aboutthe year 1845, when the bed of the Rhine was deepened arti-ficially by the blasting and removal of rock in the narrowsat Bingerloch, not far from Bingen, several flint hatchets andan extraordinary number of iron weapons of the Romanperiod were brought up by the dredge from the bed of thegreat river. The decomposition of the iron had caused muchof the gravel to be cemented together into a conglomerate.In such a case we have only to suppose the Rhine to deviateslightly from its course, changing its position, as it has oftendone in various parts of its plain in historical times, and then,when the old channel had been silted up, tools of tbe stoneand iron periods would be found in gravel at the bottom,with a great thickness of sand and overlying loam depositedin them.

Changes in a river plain, such as those above alluded to,give rise frequently to ponds, swamps, and marshes, markingthe course of old beds or branches of the river not yet filledup, and in these depressions shells proper both to runningand stagnant water may be preserved, and quadrupeds may