remembered that such alluvial terrace or bench soils differ in some respects from 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. [7] Geikie, “Text-book of Geology,” 3d ed. 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 slope-soil may often be found alternating with the alluvial deposits, or bodily washed away to be redeposited as alluvium at a greater or less distance. One characteristic of the flood-plain lands of all the larger rivers, and more or less of all streams subject to periodic overflows, is that the land immediately adjoining the banks is both higher and more sandy than are the lands farther back from the stream. The cause of this phenomenon is that as lateral overflow diminishes the velocity of that flow, its coarser portions are deposited near the river banks, while the finer particles are carried farther away, until finally only the finest—clay-substance—reach the lagoons or lakes filled with the overflow or backwater, and are there in the course of time deposited as heavy clay “swamp” soils. The same occurs where rivers empty into lakes or the sea; and these slack-water or delta lands are, as a rule, the most productive on the river’s course. The continued productiveness of alluvial soils is moreover in many cases assured by the deposition, during overflows, of fresh soil-material brought down from the head waters of the streams. The Nile, and the Colorado river of the West, illustrate this point. _Lowering of the land-surface by soil formation._—It is evident that the soil-forming agencies must in the course of time materially affect both the surface conformation and the absolute level of the land. The sharp pinnacles and crests of rock are abraded into the rounded forms now characterizing our uplands and lower ranges of hills and mountains; and it is estimated that, _e. g._, the general level of the drainage basin of the Mississippi river is lowered about one foot in 7,000 years, the material being carried into the lowlands and the sea. CHAPTER II. THE CHEMICAL PROCESSES OF SOIL FORMATION. _Chemical Disintegration, or Decomposition._ It may be said that in general, the physical agencies of disintegration are most intensely active in the dry or arid regions of the globe, while chemical processes of decomposition are most active in humid climates.