184 ' DYNAMICAL GEOLOGY. 



of the axial line, causes it to strike and erode the bank in front and deepen 

 the water, and to transfer the sand or earth removed by the erosion to the 

 opposite bank of the stream for a sand-flat ; and it thus commences a curve 

 in its course, which may become a deep bend ; aud this bend may continue 

 the action and be the occasion of a succession of such windings. The length 

 of the Mississippi between the mouth of the Ohio and the head of the passes 

 at the Gulf of Mexico is 1080 miles, while the actual distance in a straight 

 line is about 500 miles. Cutting off a bend to shorten the distance along 

 the stream increases at the place the pitch, and thereby the velocity, and 

 gives the waters greater eroding power. The flow, consequently, would 

 deepen the channel. But it is likely also to erode the banks, and may carry 

 away all the farming land the cut was intended to gain or make accessible. 

 During great floods, a stream may cut off one or more of its bends, as has 

 happened in the Mississippi, along which narrow loop-form lakes and dry 

 channels have thus been made. 



Many examples are on record of gorges, hundreds of feet deep, cut out of 

 the solid rock by only two or three centuries of work. Lyell mentions the 

 case of the Simeto, in Sicily, which had been dammed up by an eruption of 

 lavas in 1603. In two and a half centuries, it had excavated a channel 50 

 to several hundred feet deep, and in some parts 40 to 50 feet wide, although 

 the rock is a hard solid basalt. He also describes a gorge made in a deep 

 bed of decomposed rock, three and a half miles west of Milledgeville, Ga., 

 that was at first a mud-crack a yard deep in which the rains found a chance 

 to make a rill, but which in 20 years was 300 yards long, 20 to 180 feet wide, 

 and 55 feet deep ; and Liais describes a similar gorge, of twice the length, in 

 Brazil, made in 40 years. 



5. Eddies, Pot-holes, Kettle-holes. — Flowing water gathers into its current 

 any still waters alongside, to fill the void behind, which the flow tends to pro- 

 duce, and thus eddies and eddy currents are made. When alongside of a rapid 

 current, any obstruction or shallowing causes there a diminished velocity; 

 eddies become whirls, and the whirling waters bear around stones which 

 abrade the rock beneath — new stones being carried in to replace old ones as 

 they wear out. This kind of boring often goes on with hardly more change 

 of center than in a carpenter's work with his augur, and deep cylindrical holes 

 have been bored into the hardest rocks. Under a waterfall a broad basin 

 may be excavated in like manner. Pot-holes are usually from 1 to 6 feet in 

 diameter, and 2 to 20 feet deep. 



Kettle-holes are nearly circular basin-like holes 50 to 150 feet and more 

 in diameter, in stratified or unstratified sands, gravel, or drift. For some 

 reason they have failed to become filled up to the level of the region around. 

 "With regard to some, at least, of those in stratified terrace formations (see 

 page 299), the facts appear to indicate that the spots were originally holes 

 of moderate size and depth in the surface beneath ; and that in the rush over 

 the spots by the flood waters that deposited the stratified material, the 

 waters kept them free of detritus by the whirl occasioned by the depth. 



