MECHANICAL WORK OF WATER. 433 



Abrasion by wind, and especially by ice, may give a finer subdivision 

 of material than abrasion by water. For instance, it is well known that 

 glacial debris varies in coarseness from great bowlders to a silt so fine that 

 the streams issuing from the glaciers are of whitish color, and it takes many 

 days for the exceedingly finely divided material to settle when a stream has 

 passed into a lake. Chamberlin and Salisbury found, as a matter of obser- 

 vation, that the particles of loess of the Mississippi Valley smaller than 

 0.1 mm. in diameter are angular. 11 This accords with the conclusion of 

 Daubre'e that particles smaller than 0.1 mm. in diameter are not reduced 

 in size by their mutual trituration, since the particles of the loess have 

 been carried for long distances in water. However, the particles as origi- 

 nally formed by glacial abrasion may be very much smaller than this. 

 Indeed, Chamberlin and -Salisbury state that only 1.3 per cent of the 

 particles of loess of the Mississippi Valley are as large as 0.005 mm. in 

 diameter, and that probably 90 per cent of the particles do not reach half 

 this size. 6 The loess of the Rhine gave similar results, there being here 

 only seven-tenths of 1 per cent of material over 0.01 mm. in diameter, only 

 3 per cent between 0.005 and 0.01 mm., while 85 per cent is less than 

 0.0025 mm." The lower limit of size of the particles of the loess are not 

 given by any of these writers, but nothing is said which intimates that it 

 goes below the limit of visibility of the microscope. 



It thus appears tolerably certain that, however fine the subdivision by 

 any of the mechanical agents, the finest subdivision of material is only 

 accomplished by chemical agents. 



For instance, in the residuary soils of the driftless area of Wisconsin — 

 material largely subdivided by chemical agents — less than six-tenths of 1 

 per cent of the particles are as large as 0.005 mm. in diameter, less than 1.5 

 per cent are between 0.005 and 0.00285 mm., and about 98.5 per cent are 

 smaller than 0.00285 mm/ In this case much of the material is so finely 

 divided that it is stated that "since the visible proportion of exceedingly 

 fine particles was greatly increased with each increase of magnifying power, 



« Chamberlin, T. C, and Salisbury, K. D., The driftless area of the upper Mississippi Valley: 

 Sixth Ann. Kept. U. S. Geol. Survey, 1885, p. 246. 

 b Chamberlin and Salisbury, cit, pp. 279-280. 

 c Chamberlin and Salisbury, cit., p. 280. 

 ^Chamberlin and Salisbury, cit., pp. 248-249. 



MON XLVII — 04 28 



