ALTERNATIONS OF CLAY WITH POROUS STRATA. 417 



lets, which, by farther accumulation 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 ser- 

 vice in the economy of the Globe belong not to the province 

 of the geologist. Our task is limited to the consideration of 

 the mechanical arrangements in the solid materials of the 

 Earth, by means of which they co-operate with the Atmos- 

 phere, in administering to the circulation of the most impor- 

 tant 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, with strata of 

 clay that are impermeable by water;* the second circum- 

 stance is the Dislocation of these strata, resulting from Frac- 

 tures 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 subterra- 

 neous sheet of water, which is easily penetrated by wells, 

 that seldom fail except in seasons of extreme drought. The 

 accumulations of this water are reheved by Springs, over- 

 flowing from the lower margin of each bed of gravel. 



A similar result takes place in almost all kinds of per- 

 meable strata, which have beneath them a bed of clay, or 

 of any other impermeable material. The Rain water de- 

 scends and accumulates in the lower region of each porous 

 stratum next above the clay, and overflows in the same 



* See p. 62. 



