WATER AS A MECHANICAL AGENT. 169 



The coarser grains transported, by the water suffer the most in cor- 

 rasion, a grain a tenth of an inch thick wearing 10 times as fast as one 

 a hundredth of an inch, and an inch pebble losing more in transportation a 

 few hundred yards than a grain of sand of a thousandth of an inch in drift- 

 ing for 100 miles (Sorby, 1880). Angular fragments of granite lose more 

 by corrasion than rounded fragments. Ordinary sand-grains become rounded 

 in a similar manner ; but those of the finest quartz-flour from glaciers (as 

 that giving the milky tint to the Rhine at Strassburg) remain angular, 

 instead of becoming corraded (Daubree, 1879). 



Shales and soft sandstones yield easily to abrading agents ; hard sand- 

 stones and quartzytes much less so ; basalts, granites, very slowly, unless the 

 wear is promoted hf previous decay. Limestones are eroded easily because 

 the material is soft and the waters may dissolve as well as wear away. 



Abrasion assorts in proportion to hardness. The softer materials first 

 yield, leaving the harder. When granitic sands, made of quartz, mica, and 

 feldspar, are exposed to beach or river action, the mica first floats off, because 

 in thin scales ; next the feldspar is reduced in the corrasion to fine earth and 

 is borne away ; and the hard quartz is left in grains. Thus at the same time, 

 out of the same sand are made a bed of quartz sand, for a sandstone, and not 

 far off it may be an argillaceous or mud-like bed, good for forming a shale. 



Rivers and beaches are thus ever at work when materials of the right kind 

 are at hand. Where the flood-waters of a river, or the tidal-waters of the 

 ocean, flow widely over shelving shores and bordering flats with little depth, 

 the surface water as it moves onward is like a horizontally cutting blade ; 

 and, while admitting of deposition up to its level, it shears off the surface 

 with remarkable evenness, making, by this process of planation, flat shore- 

 platforms and flood-grounds or terraces, such as occur along many river val- 

 leys and sea borders ; and the plains are often at heights which make them 

 evidence of ancient water levels. 



Transportation and Deposition. 



The rate of denudation depends largely on the velocity of the transporting 

 water. The transporting power increases as the sixth power of the velocity 

 (Hopkins, 1844). With twice the velocity the weight of transportable par- 

 ticles is increased 64 times ; or, if the particles are of the same specific 

 gravity, the transportable particles, if the velocity is doubled, may have four 

 times the diameter, or 64 times the weight. 



The stones, unless they have the specific gravity of water, are moved 

 mainly along the bottom ; and being continuously under the action of gravity, 

 the movement of each, like that of a projected cannon-ball, is in a long 

 curve. It makes a series of leaps, rising from the bottom and returning to 

 it, — the length of the curve varying with the velocity and the specific 

 gravity. The finest of sediment remains long in suspension, giving a cloudi- 

 ness to waters ; and it has been suggested that a partial alteration of the 



