ALTERNATIONS OF CLAY WITH POROUS STRATA. 559 
with strata of clay that are impermeable by 
water ;* the second circumstance is the Disloca- 
tion of these strata, resulting from Fractures and 
Faults. 
The simplest condition under which water is 
collected within the Earth, is in superficial beds 
of Gravel which rest on a sub-stratum of any 
kind of Clay. The Rain that falls upon a bed of 
gravel sinks down through the interstices of the 
gravel, and charges its lowest region with a sub- 
terraneous sheet of water, which is easily pene- 
trated by wells, that seldom fail except in seasons 
of extreme drought. The accumulations of this 
water are relieved by Springs, overflowing from 
the lower margin of each bed of gravel. 
A similar result takes place in almost all 
kinds of permeable strata, which have beneath 
them a bed of clay, or of any other impermeable 
material. The Rain water descends and accu- 
mulates in the lower region of each porous 
stratum next above the clay, and overflows in the 
same manner by perennial springs. Hence the 
numerous alternations of porous beds with beds 
impenetrable to water, that occur throughout the 
entire series of stratified rocks, produce effects 
of the highest consequence in the hydraulic con- 
dition of the Earth, and maintain an universal 
system of natural Reservoirs, from which water 
* 
See pp. 70, 71. 
