210 



GEOLOGY 



Summary 



All in all, ground-water is to be looked upon as a most important 

 geological agent. When it is remembered that a very large part 

 of all the water which falls on the surface of the earth, either in the 

 form of rain or snow, sinks beneath the surface; that much of it 

 sinks to a great depth; that much of it has a long underground 

 course before it reappears at the surface; that it is everywhere 

 and always active, either in subtracting from the rock through 

 which it passes, in adding to it, in effecting the substitution of one 



Fig' 170. A sinkhole of recent development near Meade, Kan. (Johnson, 



U. S. Geol. Surv.) 



mineral substance for another, or in bringing about new chemical 

 combinations; and when it is remembered that these processes have 

 been going on for untold millions of years, it will be seen that 

 the total result accomplished must be great. The rock formations 

 of the earth to the depths to which ground-water penetrate* arc 

 to be looked upon as a sort of chemical laboratory through which 

 waters are circulating in all directions, charged with all soils of 

 mineral substances. Some of the substances in solution arc de- 

 posited beneath the surface, and some are brought to the surface 

 where the waters issue. Much of the material brought to the 1 sur- 

 face in solution is carried to the sea and utilized by marine organ- 

 isms in the making of shells. Without the mineral maticr 



