WATER IS AR-tLESTED BY IMPERVIOUS BEDS. 



313 



water that falls on the surface will readily find a passage downwards to 

 a considerable depth, and to the same cracks the water that lodges 

 among the un fissured rocks from D to E will also gradually make its way. 



The practical effects of these several conditions on the drainage of a 

 country are very obvious. If the stratum (1) be impervious to wafer, 

 the surface from A to B may be full of water, and may urgently de- 

 mand the introduction of drains, whereas if (1) and (2) be porous, tho 

 surface water will gradually sink, and the apparent necessity for artifi- 

 cial drainage will become much less striking. On tlie other hand, 

 where ihe rocks are filled with frequent cracks< as from B to C, the 

 surface water may descend and disappear so rapidly, as to render 

 useless the sinking of wells — and, as in dry summers, greatly to retard 

 the progress of the crops, or even seriously to injure the produce of the 

 harvest. Tn such a fissured state are the magnesian lime-stone rocks in 

 some parts of the county of Durham — and such is the consequent scar- 

 city of water, on some farms, that when, in long droughts, the supply 

 })reserved in artificial tanks begins to fail, the cattle must be driven to 

 water sometimes for miles, to the nearest living brook. 



2^. But water often finds its way to greater depths without passing 

 through the superior strata, and even where they are absolutely impervi- 

 ous to the rains that fall upon them. Thus along the country from A to 

 B, and especially towards A, the surface soil rests upon the upper edges 



of ihe strata. Suppose now the beds 1, 2, 3, to be impervious to water, 

 the rain tliat falls wherever these rocks lie immediately beneath the sur- 

 fnce will either remain stagnant, or will flow off" by some natural drain- 

 age. Thus from the highest point C in the above diagram, the water 

 will descend on either hand towards a and b. At h it may remain stag- 

 nant, for it cannot descend through the bed (2), which forms the bottom 

 of the valley, and the same is true of the hollow c, in which other por- 

 tions of the water will rest. All this tract of country, therefore, will be 

 m-ore or less cold, wet, and consequently unproductive. But let the bed 

 (4), the edge (or outcrop) of which forms the surface at a, be porous or 

 permeable, then the water which falls upon that spot or which descends 

 from the higher grounds about C and A, will readily sink and drain ofT, 

 descending from a towards d along the inclined bed till it finds an outlet 

 'n the latter direction. 



Thus it may readily happen that a naturally dry and fertile valley, as 

 at rt, may exist at no great distance from others, h and c, which are 

 marshy and insalubrious, and in which artificial drainage alone can de- 

 velope the agricultural capabilities of the soil. It appears also that, 

 though in any district the rocks which lie immediaiely beneath the sur- 

 face may contain no water, and may allow none to pass through them, 

 yet that other beds, perhaps at a great depth beneath, may contain much. 

 It is, in fact, this accumulation of water beneath impervious beds that 



