I4 SOILS. 



crease, first coarse and then fine sand will be the prominent 

 component of the deposited soil ; while still lower down, in 

 the region of slack water, the finest sand or silt, together 

 with clay, will predominate. According to Hopkins, 1 flowing 

 water will, at a velocity of three inches per second, carry in 

 suspension only fine clay (and silt) ; at eight inches it will 

 carry sand as large as linseed. At one and one-third inches, 

 it will more pebbles one inch in diameter; and at a velocity of 

 two inches per second, pebbles of egg size are moved along the 

 stream bed. Since the velocity of streams subject to freshets 

 will vary greatly from time to time, deposits of very different 

 grain will in such cases be found alternating with one another 

 in the soil stratum of the flood plain. In fact, this alternation 

 and the more or less stratified structure resulting therefrom, 

 is the distinguishing mark of alluvial soils as such. It is true 

 that this peculiarity is also sometimes found in the case of 

 lands now lying far above the flood-plains of present rivers ; 

 but this is due to the elevation of the land or the depression 

 of the river channels at a former period, prior to which such 

 lands (commonly known as river terraces, benches or second 

 bottoms) were formed. The same is true of lake terraces 

 (" mesas "). which cover enormous areas in some parts of the 

 world, more particularly in western Xorth America. It must 

 nevertheless be remembered that such alluvial terrace or bench 

 soils differ in some respect- fn in the modern alluvials, on 

 account of their long exposure to atmospheric action alone; 

 one result of which is that they are usually much poorer in 

 humus, and therefore of lighter tints, than the more modern 

 soils of alluvial origin. Other differences will be adverted 

 to hereafter. 



As a matter of course the above distinctions, especially 

 between colluvial and alluvial soils, cannot be rigorously 

 maintained in all cases. There are transitions from one 

 class to the other, so that it is sometimes optional with the 

 observer to which of the two classes a particular soil may 

 be considered as belonging. On the lower slopes of the hills 

 bordering alluvial valleys the colluvial slojx?-soil may often 

 be found alternating with the alluvial deposits, or bodily 



1 Geikie, " Text-book of Geology, 3d ed. 



