62 



PHYSICAL GEOLOGY 



Desert Limestone. — In arid regions the underground water may, by capillarity, 

 bring to the surface large quantities of lime which, upon evaporation, is deposited 

 as desert limestone. About Valencia, Venezuela, for example, the underlying rock is 

 almost entirely hidden by thick layers of this deposit, and extensive areas of New 

 Mexico, Arizona, and other states are covered by this limy incrustation. 



Mechanical Work of Ground Water. — The mechanical work of 

 underground water is important in producing landslides (p. 73), but 

 aside from this the effect is usually slight, since its movement is, 

 for the most part, extremely slow. 



An interesting result of the drying out of underground water was observed in 

 England at the end of a prolonged drought in the summer of 191 1. It was found that 

 the foundations of hundreds of houses which rested on clay began to settle after the 

 return of the rains. In ordinary summers the clay is quite moist at a depth of 2.5 to 

 3 feet below the surface, but during the summer mentioned it was often dry at depths 

 of 5 to 6 feet. The dry clay became powdery, and when the autumn rains began the 

 water found its way into the fissures and washed out the clay, causing sliding and 

 lateral movements. 



Springs 



The rain water which sinks into the soil and rocks through joints, 

 fissures, and pores usually issues once more to the surface through 

 seepage and springs (Fig. 37). 



Origin of Springs. — (1) Springs commonly owe their existence to 

 the presence of a stratum of pervious material overlying an impervious 



I • • 37- — -Thousand Springs, Snake River, Idaho. (U. S. Geol. Surv.) 



