630 DYNAMICAL GEOLOGY. 



illary attraction. In the drying of the soil during dry seasons, the loss 

 of moisture at the surface is attended by a rise of moisture, through 

 capillary attraction, from the deeper part of the soil ; and thus vege- 

 tation is often sustained through a long drought. If the waters below 

 contain soluble saline substances, these salts are brought to the surface, 

 there to crystallize, and make what are called efflorescent crusts ; and, 

 in a dry climate, like that of Nevada and many other regions, such a 

 crust may become quite thick, or in a lake basin, make the waters 

 saline. The saline substances referred to include common salt, carbo- 

 nate of soda, sulphate of soda, alum, sulphate of magnesia, borax, gyp- 

 sum, carbonate of soda-and-lime (gay-lussite), etc. Many remarkable 

 facts connected with Nevada deposits are described by King in his 

 Geological Report (1878). 



When infiltrating waters cause the superficial decomposition of a 

 rock, the drying of the surface tends to bring whatever is dissolved to 

 the surface, and thus produce a film over it. Limestone, if it contains 

 any iron, is sometimes covered in this way with a brownish-yellow film, 

 or, if manganese, a black film of the oxyd of manganese. 



3. Gravitation. — In the case of slopes made by the fall of dry 

 sand or stones, gravitation and friction are the chief causes of the posi- 

 tions assumed. Such an accumulation at the foot of a bluff is called a 

 talus ; and those of volcanic cinders, about a vent of eruption, make a 

 volcanic cone. A talus of dry sand may have an angle of 32° to 35°, 

 it slipping easily if at a higher angle ; one of angular stones, such as 

 forms at the base of a bluff of trap, 38° to 40° ; volcanic cinders about 

 40°. Where abundant waters from rains accompany the fall, the 

 slope, if the material is earth or sand, will be diminished to different 

 angles, from 30° to 15°, according to the amount of water ; and, with 

 a very free supply, to a much smaller angle. 



Gravity or pressure often causes compression and displacements of 

 soft beds ; and sometimes, when hard portions are inclosed in the soft, 

 the latter are pushed down along side of the former and make the ver- 

 tical groovings called Stylolites. (Marsh.) » 



III. THE ATMOSPHERE. 



The atmosphere performs geological work mechauically, (1) by 

 rending or abrading, and (2) by transportation. 



The force of the wind, measured by the pressure on a square foot, 

 increases with the square of the velocity. At 5 miles an hour, the pres- 

 sure is about 2 ounces to the square foot; at 10 miles, which is that of 

 a light breeze, 8 ounces ; at 20 miles, a good steady breeze, 2 pounds ; 

 at 40 miles, a strong gale, 8 pounds; at 60 miles, 18 pounds; at 100 



