366 STUDIES FOR STUDENTS 



a pebble i inch in diameter would be worn relatively more by being drifted a 

 few hundred yards than a sand-grain toVtt of an inch in diameter would be by 

 being drifted for a hundred miles. So long as particles are borne along in sus- 

 pension, they will not abrade each other, but remain angular. Professor Daubree 

 found that the milky tint of the Rhine at Strasburg in the months of July and 

 August was due, not to mud, but to a fine angular sand (with grains about 2V 

 millimetre in diameter) which constitutes ttoVutt of the total weight of water. 

 Yet this sand had travelled in a rapidly flowing, tumultuous river from the Swiss 

 mountains, and had been tossed over waterfalls and rapids in its journey. He 

 ascertained also that sand-grains with a mean diameter of -iV mm. will iloat in 

 feebly agitated water; so that all sand of finer grain must remain angular. The 

 same observer noticed that sand composed of grains with a mean diameter of 

 \ mm. and carried along by water moving at a rate of i metre per second is 

 rounded, and loses about tt7oo 0^ of its weight in every kilometre travelled.^ 



It is to be concluded from these statements that long transportation 

 by reducing the size of the coarser fragments and dissolving much 

 of the soluble matter tends to cause the material derived from moun- 

 tainous regions or sub-arid climates finally to approach the chemical 

 and mechanical nature of the waste derived from topographically 

 gentler or climatically more humid regions. This conclusion is con- 

 firmed by observation of the beds of rivers. 



Walther shows that the large rivers carry very different sediment 

 in the different portions of their courses. 



In the upper course the deposits show a complete assemblage of all kinds of 

 rocks and minerals found within the reach of the river system. In the middle 

 course the pebbles disappear, but the sand is still of various kinds and contains 

 besides the original constituents, contributions from the neighboring streams. 

 The first materials to disappear are those minerals soluble or readily disintegrated, 

 but hardness is also important in exercising a selective action, since in the rubbing 

 of stones together the softer are soon destroyed. In the upper coiu"se of the Avisio 

 a great quantity of limestone pebbles may be detected between blocks of porphyry 

 and syenite. In the lower valley, however, the limestone pebbles completely 

 disappear and the eruptive rocks increase relatively in number. Consequently 

 in the erosion of a mountain consisting of a schistose formation containing second- 

 ary quartz veins the entire matrix may by the transportation be transformed into 

 fine clay and the quartz pebbles alone remain as the indestructible residue.^ 



Walther further states that 

 it may be directly observed upon the bed of the Rhine that sand and small frag- 

 ments will be moved forwards several decimeters while a larger fragment lying 



1 A. Geikie, Textbook of Geology, 1903, pp. 496, 497. 



2 Translated from Einleitung in die Geologie (1893-94), p. 758. 



