GEOLOGY. 257 
of all the masses of stone so precipitated into the lower 
lying channel? In taking this matter into consideration, 
we are brought upon the consideration of the transporting 
power of water, which has been previously referred to. 
The rock, precipitated on to the moving glacier, must 
move on with the moving mass of ice; and so within 
certain limits is it with the sand and gravel borne along 
by a river. Most are familiar with the appearances pre- 
sented by deposits in a river bed after a flood, which has 
passed away and left the river bed in many places dry: 
here shingle, here gravel, here sand. M. Costa de Bastelica, 
in a work entitled Aapport au Conseil Federal. sur les 
Torrents des Alpes Suisses Inspectés en 1858-1863, published 
at Lausanne, gives some information on this subject. He 
embodies in his idea of a torrent its bearing along earthy 
matter in suspension ; and he states that 1t does so both 
in a mass and in what is known in France as triage, drop- 
ping some and carrying on others of the materials in 
question ; in the former case all the rocks, pebbles, and 
lesser fragments are carried along in something like their 
relative positions, as would be the case in a viscid mass or 
in a glacier; in the latter the weightier materials are 
dropped first, and this going on more or less continuously, 
the matters in a state of extreme comminution are carried 
furthest. The difference in mode of transport appears in. 
connection with difference in the velocity of the flow. 
When this is so great as to bear the whole along in a mass, 
the stones, whatever their size, do not come into collision, 
and if any were withdrawn they would be found to be as ° 
little rounded as are the stones falling from a glacier, and 
forming a moraine. 
But when the velocity is being impaired, as this goes on 
the stones begin to roll, suspended in the water, when they 
may come into collision one with another ; and the heavier 
sinking, these are for a time are rolled along the bottom, and 
subjected to collision and friction. At length they rest, 
and where they rest the collision of others following and 
proceeding further subjects them to continued abrasion ; 
iS) 
