206 
POPCLAE SCIENCE REVIEW. 
the penetration of water for any depth into the earth. In the 
illustration to which reference has already been made, we have 
the most favourable condition for subterranean drainage that 
can occur, viz.: a porous stratum overlying and overlaid by 
impermeable ones, as, for instance, the Lower Green Sand 
lies between the Gault and the Wealden Clay. The rain 
below that portion of the hill (c) where the Lower Green 
Sand crops up will not penetrate below the layer of alluvial 
soil, by which the earth is thinly covered, but wnll gradually 
make its way down the incline to the level of the valley at e, 
where it will form a lake or river, as the case may be. This is 
shown still better in the second illustration, which represents 
a section of a hill formed of unstratified rock, covered with a 
thin coating of soil. The rain-fall on such a hill will gradually 
soak down its sides to appear as a spring at Q on the right ; 
or, if a portion of the surface has been denuded, as a stream, 
at Q on the left. But it does not follow because the strata 
of a given formation are, in the main, composed of a rock that 
is itself impermeable that they are therefore incapable of 
drainage. It is quite possible for water, under such circum- 
stances, to find its way through the seams which separate the 
different members of such a formation, and which are often 
filled with sand or some other porous material. By referring' 
to the thhd of our illustrations we shall not only see how this 
may occur, but also how it may affect the hydrographical 
features of a locality. Here v\^e have a hill formed of stratified 
rock, which has been so disturbed as to give it a dip to one 
side. The rain which falls on the left side of the hill will find 
its way downwards through the seams of stratification until it 
meets with the drainage of the other side, which, in consequence 
of the peculiar direction of the stratification, will be confined 
to the surface, and the two will give rise to a stream or spring 
at Q. We have a further proof of the influence of geological 
disturbances on the water supply of a district in the first of 
our illustrations, which represents a valley (t), which has been 
formed by the disruption of a stratified rock (a) in consequence 
of the projection through it of a mass of volcanic rock. It 
will be seen that owing to the dip of the strata on either side 
being outwards, the rain which falls on the sides of the valley 
will drain off to the opposite side, where it will form streams 
or springs at Q, whilst the valley itself will be destitute of such 
features. 
It often happens that where the superficial drainage of a 
district is insufficient to provide a water supply equal to the 
local demands, an attempt is made to reach the reservoirs of 
water which are supposed to exist in the strata at a consider- 
able depth below the surface, by boring into them until water 
