216 GEOLOGY OF THE HIGH PLATEAUS. 



we may note the stones which pave its bed, and after a flood has passed 

 and the stream again is clear we may find that there has been Httle change 

 in tliem; but to conclude that no stones have passed in the interval would 

 be a mistake. Those which retain their places have lodged there and been 

 fastened to the bottom by a packing of sand or wedged together like the 

 cobbles of a pavement. If the sources of the materials continue to furnish 

 them, doubtless many stones have been hurried along over this pavement 

 during the flood, a few finding a resting place, but more of them passing 

 on to be ground into silt or to find resting-places in deeper waters below. 

 But there is another method quite different from this precipitate one, 

 and by which it is very probable that much larger movements are eff"ected, 

 though much more slowly. It never happens that the materials to be 

 moved are of uniform grain. Mud, sand, gravel, shingle, and cobble- 

 stones always accompany coarser debris in varying proportions, and form a 

 matrix in which the larger fragments are imbedded. An acceleration of 

 the current removes the finer stuff and retardation replaces it with fresh. 

 The washing out of the matrix of sand and grit which holds a pebble in 

 its place leaves the pebble to the unobstructed energy of the current. If 

 that energy is sufficient it will be carried along until the current slackens 

 or until it finds a lodgment. If the energy is too small, the pebble will 

 remain until the ceaseless wear of attrition reduces it and brings it within 

 the power of the stream to move it. Nor are these movements dependent 

 solely upon periodical floods. Any cause which alternately accelerates the 

 movement of water may produce them, and these causes are many. Every 

 stream and every shore cuiTent is affected by numerous rhythmical move- 

 ments which produce these alternations in many ways and many degrees. 

 The waves and surf, the undertow, the tides, the shifting of shore cur- 

 rents, the stomis and monsoons, the ripples of the brook, the numberless 

 surgings and waverings of rivers, the shifting of channels, the building and 

 destruction of sand bars, the freshets — all are causes by virtue of which 

 any spot at the bottom of the water is subject to alternate maxima and 

 minima in the velocity of the water which passes over it. Sooner or later, 

 then, the pebble must move on, provided any maximum of velocity in 



