398 Notes on the Formation of extensive 



gle beaches as tideless seas ; and as most of them are lower 

 at one time than another, we may observe the shingle beaches 

 better; and it is by no means uncommon to see a skirting oi 

 shingle round them when their waters are low. 



III. Action of Rivers on their Beds. 



Rivers most frequently, though not always, take their rise 

 among hills and mountains, and are supplied either by the 

 melting of snows or glaciers, the draining of rain waters, or by 

 springs. The two former particularly bring down fragments 

 formed by discomposition from the neighbouring rocks, into 

 the bed of the river. In mountainous regions fragments of 

 rocks of greater or less dimensions fall into the river from the 

 mountain sides. The river also undermines its banks, and the 

 loose decomposed surface of the rocks tumbles into it. From 

 these sources the river obtains the materials for its gravel. 

 The greater the velocity of the water, the sooner will the angu- 

 lar fragments be ground by attrition into pebbles. Rivers are 

 most rapid in high mountain ranges, having to find their way 

 from a high to a much lower level in comparatively short dis- 

 tances. Now as the decomposition and the fall of rocks is 

 greatest amid high mountains, and as the rivers are most rapid 

 in the same situations, the greatest quantity of river gravel is 

 there produced. 



In low situations, where rivers lose their rapidity, gravels 

 are rarely formed, but sands or mud are common. In times of 

 flood, gravels formed in their beds, in the high lands, are 

 brought down into their beds in the plains; but even these do 

 not appear to travel far. It is asserted, but has not been 

 proved, that rivers carry their gravels to considerable distances ; 

 but I cannot avoid suspecting that pebbles derived from great 

 gravel plains, or from cliffs of loosely aggregated conglome- 

 rate, such as the Nagelfluhe of Switzerland, cut away by the 

 rivers, and thus carried into their beds, have been Sometimes 

 mistaken for gravels transported from great distances by the 

 rivers. There is no want of gravel, composed of pebbles from 

 the high Alps, in the bed of the Rhone, where that river quits 

 the lake of Geneva, or in the bed of the Ticino, where it quits 

 the Lago Maggiore ; and I presume no person would imagine 

 that the gravels have been brought down by either river from 

 the Alps, as all such pebbles must have been quickly deposited 

 in the bottoms of the respective lakes. In both instances the 

 gravels have been derived from conglomerates formed by more 

 general causes, cut through by therivers after they have quitted 

 the lakes. Innumerable other instances might be produced. 

 The same observation applies to rivers cutting great gravel 

 plains, where they obtain pebbles, derived originally from dis- 



