CONDITIONS OF SEDIMENTARY DEPOSITION. 477 



Products: Rock cores of disintegrated masses, sand, (chiefly 

 quartz-sand), residual clays, and lime, magnesia, 

 iron, etc., in solution. 



TRANSPORTATION. 



Favorable co?iditions: 



[a) Steep slopes. 



(&) Abundant rainfall. 



(c) Absence of vegetation. 



(d) Floods 



(<?) Fine detritus. 



By comparison of the statements of favorable conditions for 

 rock-breaking, rock-decay and transportation it becomes apparent 

 that breaking and decay are favored by opposite conditions in 

 nearly all respects, while breaking and transportation are most 

 efficient under like conditions. But breaking promotes decay, 

 and decay aids transportation, by reducing the size of the parti- 

 cles to be decomposed and carried, and the maximum effect of 

 erosion is probably attained when rock-breaking is active 

 among greater elevations, and rock-decay and transportation are 

 both proceeding energetically on lower slopes. * 



The amount of material furnished by erosion is an important 

 consideration in reference to the rate of accumulation of sedi- 

 ments over a given area, and is a condition not to be overlooked 

 in comparing thicknesses of deposits with the lapse of geological 



ages. 



SEQUENCE OF SEDIMENTS. 



Shingle, gravel, sand, clay and silt are products of ero- 

 sion of rock masses. They are produced either by mechanical 

 breaking or by chemical disintegration. These two sub- 

 processes of the general process of erosion are favored by 

 unlike conditions. Those conditions which render breaking 

 most efficient are unfavorable to immediate disintegration ; and 

 those conditions which promote disintegration limit breaking. 

 Breaking, the reduction of a rock mass to small pieces, is usually 



1 Gilbert, Henry Mts. p. 105. 



