s 



ATMOSPHERIC AGENCIES. 



soils are often shifted, they are usually composed of a mixture formed 

 by the disintegration of several kinds of rock. In some cases the soil 

 has been formed in situ during the present geological epoch, and the 

 process is still going on before our eyes. Such are the soils of the hills 

 of the up-country or primary region of our Southern Atlantic States.* 

 Sometimes the soil formed in the same way has been shifted to a greater 

 or less distance. Such are the soils of our valleys and river-bottoms. 

 In still other cases the soil has been formed by the process already de- 

 scribed, and transported during some previous geological epoch and not 

 reconsolidated. Such are many of the soils of the Southern low-country 

 or tertiary region. 



Mechanical Agencies of the Atmosphere. 



Frost. — Water, penetrating rocks and freezing, breaks off huge frag- 

 ments : these by a similar process are again broken and rebroken until 

 the rock is reduced to dust. These effects are most conspicuous in cold 

 climates and in mountain-regions. In cold climates huge piles of 

 bowlders and earth are always seen at the base of steep cliffs (Fig. 5). 



Such a pile of materials, the ruins 

 of the cliff above, is called a talus. 

 In mountainous regions frost is a 

 powerful agent in disintegrating the 

 rocks, and in determining the out- 

 lines of mountain-peaks. This is 

 well seen in the Alps and in the 

 Sierra. 



Winds. — The effect of winds is 

 seen in the phenomenon of shifting 

 sands. At Cape Cod, for instance, 

 the sands thrown ashore by the sea are driven by the winds inland, and 

 thus advance upon the cultivated lands, burying them and destroying 

 their fertility. The sands from the beach on the Pacific coast near 

 San Francisco are driven inland in a similar manner, and are now reg- 

 ularly encroaching upon the better soil. Large areas of the fertile 

 alluvial soil of Egypt, together with their cities and monuments, have 

 been buried by the encroachments of the Sahara Desert. The same 

 phenomena are observed on various parts of the coast of France, Hol- 

 land, and England. The rate of advance has been measured in some 

 instances. Thus on the coast of Suffolk it is said to advance at the 

 rate of about five miles a century ; at Cape Finisterre, according to 

 Ansted, at the rate of thirty-two miles per century, or five hundred and 

 sixty yards per annum. The Dunes of England and Scotland are such 



Fig. 5. 



* In the Northern States, in the region of the Drift, nearly all the soil has been shifted. 



