12 
Arterial Drainage 
permeable stratum at E would yield a constant supply until the 
natural reservoir F D was exhausted. 
The rain which falls on most volcanic and unstratified rocks 
flows so freely off the surface, and is carried so directly to the 
stream, that nothing is stored for summer supply. In the more 
recent formations the amount of percolation is so considerable, 
except in the case of the clays, that there is scarcely any flow 
off the surface. The flow of the rivers is nearly constant, and 
floods in these districts are rare, only occurring under ex- 
ceptional circumstances. 
A rainfall of an inch in ten or twelve hours will nearly all 
run off" a hill of unstratified rock or clay almost as quickly as it 
falls, but on a steep chalk or limestone hill, even at elevations 
of 800 or 900 feet above the level of the sea, although there 
might be a rainfall of 2 inches in an hour, the whole would sink 
into the ground. So also in the red sandstone the water is 
absorbed almost as fast as the rain reaches the ground, and con- 
sequently the supply of water from wells in this formation is 
always abundant.* Sand, when dry, will absorb from 2 to 3 
gallons in every cubic foot, and water will permeate as much as 
18 inches in depth in one hour.f 
The capacity of chalk soils to receive the rainfall by percola- 
tion is easily accounted for when it is considered that a cubic 
foot of chalk, when dry, will absorb from 2 to 2J gallons of 
water, or from 33 to 40 per cent, of its bulk, equal to 56,000,000 
gallons for every foot in depth per square mile, or about 4 inches 
of rainfall. I (Ansted.) 
So strong is this power of absorption in these soils, the 
chalks especially, that they may always be traced on a map by 
the absence of streams and rivers. 
The difference between the amount of absorption of chalk 
soils and those of an impervious nature is also shown by the 
size of the openings of bridges and culverts across streams run- 
ning through chalk and clay districts. From a comparison 
made by Mr. Homersham of nine pairs of bridges over the 
Thames or its tributaries, one bridge of each pair spanning 
respectively a stream draining an area of chalk, and the other 
crossing a stream draining a nearly equal area of the London 
clay, or of London clay with a little chalk, the ratios of the 
water-ways draining the chalk varied from one-tenth to three- 
tenths of the area of those draining the clay, the latter being 
* "The Water Supply of Pnislev," ' Trans. Instit. Civil Engineers,' vol. xxxi. ; 
" The Rise and Fall of the Wandle," idem, 1801. 
t ' Water Supply of Cities and Towns.' By W. Humbcr. Crosby Lockwood 
and Co. 
I " The Chnlk Water System," ' Trans. Instit. Civil Enginccrti,' vol. xlvii. 
