650 VICTOR ZIEGLER 



by the carbonates of calcium and magnesium, and a consequent 

 settling of the silt. 1 The presence of salts, alkalies, and acids in 

 solution hasten the rate of precipitation. However, Wheeler 

 arrives at the conclusion that there is practically no difference 

 in the rate of settlement of sand and silt in salt and fresh water. 2 

 When the particles are very fine, as mud and ooze, the rate of 

 settlement is slightly faster in salt than in fresh water. Others 

 have shown that settling is far more rapid in salt than in fresh 

 water, and attribute this fact to a chemical interaction between 

 the salt water and the sediment, carried in this case as a colloid. 3 

 There is reason to doubt this explanation, and the more rapid 

 settling in salt water seems to be due to a decrease in the viscosity 

 of the water. 4 Rough and irregular river bottoms and swinging 

 meanders tend to keep the water in a stirred condition and hence 

 aid in holding material. 



METHODS OP ROUNDING 



Sand grains are reduced in size by collision and friction. Hence 

 we know that the wear of a grain depends on a number of factors, 

 such as hardness, weight, distance of travel, cleavage, tenacity, 

 velocity of movement, etc. The rounding of sand grains under the 

 varying conditions has been ably discussed from the geological 

 standpoint by McKee 5 and Goodchild. 6 The movements of solids 

 through fluids have been investigated from the mathematical stand- 

 point especially by Basset 7 and Allen. 8 This feature has also 

 been noted to some extent by Blake, 9 Walther, 10 and Barrell. 11 



* x E. W. Hilgaard, A.J.S., 1873, P- 288; 1879, p. 205. 



2 W. H. Wheeler, Nature, June 20, 1901. 



3 See F. W. Clarke, Bull. 330, U. S.G.S., and H. S. Allen, Nature, July 18, 1901, for 

 bibliographies. 



4 J. F. Blake, Geol. Mag., Decade IV, Vol. X, 12; W. B. Scott, Introduction to 

 Geology, 141; Carl Barus, Bull. 36, U.S.G.S.: Chamberlin and Salisbury, College 

 Geology, 316. 



s McKee, Edinburgh Geol. Soc., VII, 298. 6 Goodchild, ibid., 208. 



I Basset, Elementary Treatise on Hydrodynamics. 

 8 Allen, Phil. Mag., 1900. 



» Blake, Geol. Mag., Decade IV, Vol. X, 12. 

 10 Walther, Das Gesetz der Wustenbildung. 



II Barrell, Jour. Geol. (1908), XVI, 159. 



