_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 pulverulent 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. 188) 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 dependent 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 proportion of cobbles and gravel is often present in the deposits, sometimes to the extent of rendering cultivation impracticable, or at least unprofitable. As the slope and velocity decrease, 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,[7] 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 _move_ 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 North America. It must nevertheless be