II.] SPRINGS. 29 



may happen to run through the upper bed. Though the 

 material of B may be as porous as a sponge, not a drop of 

 water can reach it, as long as the waterproof roof remains 

 sound. But the case is extremely different when the beds 

 are inclined, as represented in Fig. 9. Here are three 

 beds, in the same order as those previously described, but 

 dipping at a slight angle. The porous bed B is exposed 

 at the surface, or as a geologist would say " crops out." 



%■''■:■■:■ B 



Fig 8. — Horizontal permeable and impermeable strata. 



Rain falling upon the ground A B C is thrown off by 

 the two clay beds A and C, but is absorbed by the out- 

 crop, or exposed surface, of the sandy stratum B. This 

 absorbed water, whether directly falling upon B, or drained 

 off from A, runs down in the direction of the dip until 



Fig. 9. — Inclined permeable and impermeable strata. 



it reaches an oudet, whence it issues as a spring. If a 

 valley should cut through the beds, and have its bottom 

 below the water-level, springs will be thrown out along 

 the sides of the valley, as at D. 



In following the course of a set of strata, it is no un- 

 common thing for the geologist to find that they come 



