CLIMATE AND TERRESTRIAL DEPOSITS 365 



further shows that the decomposition occurring while undergoing 

 abrasion due to movement in water takes place under exceptionally 

 favorable circumstances, since if the initial film of decomposition is 

 not continually removed the action of the water rapidly slows down;'^ 

 and furthermore that the water alone, where the clogging films are 

 continually removed, is well able thoroughly to decompose feldspar 

 without the intervention of acid.^ To the extent then to which 

 mechanical abrasion occurs during river transportation decomposition 

 is also favored and takes place at a vastly more rapid rate than in the 

 normal weathering, which however persists through a far longer time 

 while the material, owing to the lowering of the surface of erosion, is 

 passing from the solid rock through the zone of soil. As to the 

 mechanical effects: 



In the series of experiments already referred to, Professor Daubree made 

 fragments of granite and quartz to slide over each other in a hollow cylinder 

 partially filled with water, and rotating on its axis with a mean velocity of 0.80 

 to I metre in a second. He found that after the first 25 kilometres (about 15J 

 English miles) the angular fragemnts of granite had lost xV of their weight while 

 in the same distance fragments already well rounded had not lost more than too" 

 to 4 oTT- The fragments rounded by this journey of 25 kilometres in a cylinder 

 could not be distinguished either in form or in general aspect from the natural 

 detritus of a river-bed. A second product of these experiments was an extremely 

 fine impalpable mud, which remained suspended in the water several days after 

 the cessation of the movement. During the production of this fine sediment, the 

 water, even though cold, was found in a day or two to have acted chemically upon 

 the granite fragments. After a journey of 160 kilometres, 3 kilogrammes (about 

 6 J lb. avoirdupois) yielded t^.^, grammes (about 50 grains) of soluble salts, con- 

 sisting chiefly of silicate of potash. A third product was an extremely fine 

 angular sand consisting almost wholly of quartz, with scarcely any feldspar, 

 nearly the whole of the latter mineral having passed into the state of clay. The 

 sand-grains, as they are continually pushed onward over each other upon the 

 bottom of a river, become rounded as the larger pebbles do. But a limit is placed 

 to this attrition by the size and specific gravity of the grains. As a rule, the smaller 

 particles suffer proportionately less loss than the larger, since the friction on the 

 bottom varies directly as the weight and therefore as the cube of the diameter, 

 while the surface exposed to attrition varies as the square of the diameter. Mr. 

 Sorby, in calling attention to this relation, remarks that a grain iV of an inch, in 

 diameter would be worn ten times as much as one too of an inch in diameter, and 



1 "The Decomposition of the Feldspars," U. S. Dept. of Agriculture, Office of 

 Public Roads, Bull. No. 28, 1907, p. 10. 



2 Op. cit., p. 14. 



