630 DYNAMICAL GEOLOGY. 



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 

 vegetation 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. This crust the rains 

 may afterward dissolve and wash away, or, if the region is a basin, 

 make by means of it a salt lake. The saline substances referred to 

 include common salt, carbonate of soda, sulphate of soda (Glauber 

 salt), alum, sulphate of magnesia (Epsom salt), borax, gypsum, car- 

 bonate of soda and lime (gay-lussite), etc. 



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 any manganese, a black film, the film in each of these cases being 

 an oxyd of the metal present. 



4. Gravitation. — Gravitation is concerned in all the mechanical 

 changes carried on over the globe, influencing the arrangement and 

 positions of material, and having a controlling action over all move- 

 ments. In the case of slopes of dry, falling sand or stones, gravitation 

 and friction are the chief causes of the positions assumed. Such an 

 accumulation at the foot of a bluff is called a talus ; and those of vol- 

 canic cinders, about a vent of eruption, 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°. Assumption 

 Island, one of the northern Ladrones, and a cone of this kind, has, as 

 observed by the author, a slope scarcely varying from 40°. Where 

 abundant waters from rains accompany the fall, the slope, if the mate- 

 rial 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. 



m. THE ATMOSPHERE. 



The following are some of the mechanical effects connected with 

 the movements of the atmosphere. 



1. Destructive effects, from the transportation of sand, dust, etc. — The 

 streets of most cities, as well as the roads of the country, in a dry 

 summer day, afford examples of the drift of dust by the winds. The 

 dust is borne most abundantly in the direction of the prevalent winds, 



