558 PERPETUAL CIRCULATION OF WATER. 
A fourth portion descends into the strata, and 
is accumulated in their interstices into subterra- 
neous sheets and reservoirs of water, from which 
it is discharged gradually at the surface in the 
form of perennial Springs, that form the ordinary 
supply of Rivers. 
As soon as Springs issue from the Earth, their 
waters commence their return towards the Sea ; 
rills unite into streamlets, which, by further ac- 
cumulation form rivulets and rivers, and at length 
terminate in estuaries, where they mix again with 
their parent ocean. Here they remain, bearing 
part in all its various functions, until they are 
again evaporated into the Atmosphere, to pass 
and repass through the same Cycles of perpetual 
circulation. 
The adaptations of the Atmosphere to this 
important service in the economy of the Globe 
belong not to the province of the geologist. Our 
task is limited to the consideration of the me- 
chanical arrangements in the solid materials of 
the Earth, by means of which they co-operate 
with the Atmosphere, in administering to the 
circulation of thi^ most important of all fluids. 
There are two circumstances in the condition 
of the strata, which exert a material influence 
in collecting subterraneous stores of water, from 
which constant supplies are regularly giving forth 
in the form of springs ; the first consists in the 
Alternation of porous beds of sand and stone. 
