SPRINGS 
175 
CHAPTER XVII. 
AGENTS WHICH FORM ROCKS (CONTINUED). 
Springs. 
Springs, how Supplied.-— Artesian Wells. — Mineral and Ther- 
mal Springs. — -Calcareous Springs. — Travertin.— Silicious 
Springs. — Geysers of Iceland.— Sinter — Hot Springs of St. 
Michael.— Silicious Deposues. — Ferruginous Springs. — Nap- 
tha and Asphaltum 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 
