656 



PRINCIPLES OF STRATIGRAPHY 



water than in lakes. Experiments (Brewer-ii : id^") have shown 

 that clay which had been in suspension in fresh water for thirty 



Fig. 130. Diagram showing the lateral shading off of the clastic sands and 

 pebbles into calcareous deposits without intervening muddy 

 phase. The successive formations a, b. and c change shoreward 

 into arenytes. An apparently continuous sand bed; a-c is thus 

 produced resting upon the old land surface. 



months had not settled out as clearly as the same clay from a solu- 

 tion of common salt in less than thirty minutes. These results have, 

 however, been questioned by Wheeler (103), who gives the accom- 

 panying table, showing the rate of settlement in the two media : 



Table Showing Rate of Settlement of Solid Matter in Fresh and 



Salt Water. 



No. 



No. of 

 grains 



to a 

 lineal 



inch 



20-60 

 100 

 200 



300 



1,400 

 500 

 1,000 

 2,000 

 600 

 1,500 

 1,600 

 1,440 



Material 



Small pebbles 



Small pebbles 



Coarse sand 



Coarse sand 



Sand 



Sand 



Whiting 



Plaster of Paris 



Warp Trent 



(Fine warp, Dutch river). 



Silt, salt marsh 



Warp marsh 



(Alluvium, Boston Dock) 

 (Alluvium, River Parrett) 



Tilbury Basin 



Brick clay 



Boulder clay 



Time taken 

 to settle 



Fresh Salt 



Water 

 clear 



Fresh Salt 



Ft. 



per 



minute 



o. 70 

 0.42 



0.46 

 0.40 

 0.35 



Remarks 



Water not 

 discolored 



do 



do 



do 



do 



do 



do 



do 

 Water scarcely 



discolored 

 Water turbid 



do 



do 



do 



do 



do 



do 



do 



The phenomena of flocculation have been attributed to chemical 

 reaction, but seem to find a better explanation in the forces of at- 



