THE PHYSICAL PROCESSES OF SOIL FORMATION. I3 



of the stream from various sources and carried to a distance 

 before being deposited, the soil is designated as alluvial. 

 These are the soils of the valleys, flood-plains, and sea- and 

 lake-borders, past and present. Being of mixed origin, their 

 general character may vary from one extreme to the other, 

 both as regards physical and chemical composition. Since, 

 moreover, they represent the finer portions of the soils of the 

 regions drained by the watercourses, alluvial soils are as a rule 

 of a fine texture ; and as representing the most advanced de- 

 composition products of the parent rocks, they are usually 

 preeminently fertile. This is proverbially true of the flood- 

 plains of rivers, and still more of their deltas the bodies of 

 lands formed near their outlets into seas or lakes. 



Character of these soil-classes. Sedentary soils are as a 

 matter of course, other things being equal, dependent entirely 

 on the parent rock for their specific character ; and taking into 

 consideration the various rocks (usually one or few) from 

 which they may have been derived, nearly the same is true of 

 colluvial soils, except that a portion of the clay and finest pul- 

 verulent matters may in their case be carried down on the 

 lower slopes and into the valleys and streams, by the hillside 

 rills. 



According to the calculation of Merrill (Rocks, Rock-weathering, and 

 Soils, p. 1 88) granite when transformed into soil without loss would 

 increase in weight by 88 % ; more than doubling its bulk. More 

 usually, the leaching process diminishes their volume as compared with 

 the parent rock. 



Alluvial soils are also of course to a certain extent de- 

 pendent upon the character of the rocks and surface deposits 

 occurring within the drainage area of the depositing stream. 

 As a rule their composition is much more generalized ; but their 

 character as to the relative proportions of sand and clay is 

 essentially dependent upon the velocity of the water current. 

 Thus in the upper portions of valleys, where the slope is 

 relatively steep and the velocity therefore high, a large pro- 

 portion of cobbles and gravel is often present in the deposits, 

 sometimes to the extent of rendering cultivation impracti- 

 cable, or at least unprofitable. As the slope and velocity de- 



