8 SOILS. 



nels and caves, defining sharply the varying degrees of hard- 

 ness or tough resistance in different parts of rocky cliffs; 

 frequently undermining them and causing extensive rock-falls. 

 The latter then serve for a time to break the violence of the 

 waves' onset, and may even cause permanent shore deposits to 

 be formed under their lee. 



Such deposits are very generally formed on gently sloping 

 beaches, and as the water gradually recedes, sometimes by 

 elevation of the ground, beach lines or beach-terraces are left, 

 which indicate the successive levels of the lake or sea. Such old 

 beach lines or terraces and level-surfaced " buttes " in the Great 

 Basin country, and 'bench lands" elsewhere, show in their 

 structure the characteristic lines of wave-deposition. 



Effects of U'inds. The action of winds in transporting soil 

 particles (dust and sand) is familiar; and the accumulations 

 that may he formed under the influence of regular, continuous 

 winds are sufficiently obvious mi lee shores having sandy 

 beaches, inland of which the formation of sand dunes at times 

 assumes a threatening magnitude. \Yhere winds are irregular, 

 frequently reversing their direction, of course the local effects 

 will be less obvious, and the transportation of material actually 

 occurring will often not be noticed. Vet there can be no doubt 

 of the importance of wind action in soil formation, and there 

 are cases in which no other agency can explain the facts ob- 

 served over widely extended areas. This is especially true with 

 regard to the soil masses of the high plains or plateaus of the 

 dry continental interiors, where not only the regularity of the 

 prevailing winds, but also the structure (or absence of struc- 

 ture) and pulverulent character of the soil itself, renders this 

 the only rational mode of accounting for its presence where we 

 find it. 



The effects that may be exerted l>v regular winds are well illustrated 

 in the plains and deserts of Africa as well as those of central Asia. 

 Here we find a distinct subdivision of the desert (rainlc-^ .-ire.is into the 

 stony, from which the wind has swept all but the bedrock and gravel ami 

 where scarcely any natural growth, and certainly no cultivation is pos- 

 sible in the almost total absence of soil. The next subdivision is the 

 sandy desert, to leeward of the stony area, where the winds are less 

 violent and regular, and where, therefore, the sand has been dropped 



