"•] 



SPRINGS. 



31 



It frequently happens that the beds of rock, instead of 

 having a uniform dip, slope first downwards and then 

 upwards, so as to assume a basin-like form, such as that 

 shown in Fig. 11. Here it is seen that the strata on opposite 

 sides slope in opposite directions, as indicated by the 

 arrows. Rain falling upon the exposed edges of the 

 porous rock B B, will be readily absorbed, and will per- 

 colate through the pervious material until it reaches the 

 bottom of the trough, where it will accumulate, and of 

 course be accessible to the boring-rod. If, therefore, a 

 bore-hole be put down through the impervious bed A, 

 it will tap this reservoir of water, and the liquid -will then 

 rise to a height dependent on the level of the water in 



Fig. II. — Diagrammatic section across the London basin. 



the bed B B. The laws which regulate the flow of watei 

 underground are precisely those laws which regulate its 

 flow above ground. The water pent up in the bed B B 

 will therefore rush up the tube, and tend to find its own 

 level. 



This arrangement of strata in Fig. ir, may be taken 

 to represent, roughly, the disposition of the rocks beneath 

 London. The beds in that area have been thrown into 

 a trough-like form, and have thus produced what is com- 

 monly known as the Londoti basin} Yet it must not for 



The London basin, or area around the metropolis, forms only a 

 small portion of the Thames basin. 7 he two are to be kept carefully 

 distinct. 



