SPRINGS 



175 



CHAPTER XVIL 



AGENTS WHICH FORM ROCKS (cOJmNUED). 



Springs. ^ 



Springs, how Supplied.— Artesian Wells. — Mineral and Ther- 

 mal Springs. — Calcareous Springs. — Travertin.— Silicious 

 Springs. — Gey.'sers of Iceland.— Sinter. — Hot Springs of St. 

 Michael.— Silicious Deposites. — Ferruginous Springs, — Nap- 

 thaand Asphaltuna Springs. — Pitch Lake of Trinidad. 



Springs are mostly supplied by water which falls 

 in rain on the surface of the earth. This is shown 

 by the fact that, after a long drought, they are apt to 

 dry up, and in wet seasons they flow most copious- 

 ly. Every person is aware that porous soils absorb 

 water with great facility, as such ground soon be- 

 comes dry after heavy showers. The depth to which 

 we have to penetrate before reaching water depends 

 in a great degree on the nature of the strata beneath. 

 Thus, if a bed of clay lies under loose sand and 

 gravel, it serves as a non-conductor of water, and, 

 of course, we shall find water probably before we 

 reach it. The same is often the case if a stratum 

 of rock lies beneath the soil, unless the shape of the 

 surface is such that the water is carried off by de- 

 scending to a lower level. As a general rule, in 

 sinking wells it is necessary to penetrate near to 

 some stratum impervious to water, before we shall 

 meet with a sufficient supply of this element ; for 

 here the water accumulates as in a reservoir, and is 

 ready to flow out into any opening which may be 

 made. This may be illustrated by the effect of the 

 tides in the Thames, near London. The river here 



