72 Rainfall, Natural Drainage, 



remarkable instances of this. On the steep flanks of a lofty moun- 

 tain, in the middle of Cephalonia, there is an accumulation of 

 loose, ans^ular fragments of limestone rock standing at the natural 

 angle. On this heap there neither is nor can be an atom of soil. 

 The rain when it falls is very heavy, and would carry down to 

 the bottom any loose particles that might be conveyed thither by 

 man or formed by nature : the rain itself also must sink at 

 once to the bottom. On this heap of bare stones the vine 

 not only grows but flourishes, and the grape ripens admirably. 

 The rootlets twine round the stones, and they manage to extract 

 from them and from the air all the nourishment they require. 

 I have often seen crops on stony ground, but nowhere so marked 

 an instance as this of the resources that exist in solid naked 

 rock under favourable circumstances. 



It is only clays and similar tenacious rocks that do not freely 

 admit of the passage of water. Even these admit it slowly and 

 imperfectly, and they always contain moisture. The amount of 

 the water contents varies according to weather, and the water 

 is here also in circulation, though slowly. These tenacious rocks, 

 however, greatly assist in the distribution and circulation of water 

 in the earth in another way. Passing freely where the soil is 

 open, water is checked in its course where the rock becomes 

 compact and impermeable. As these alternations occur some- 

 times in parallel and horizontal strata or layers, sometimes in 

 strata inclined to the horizon, sometimes in vertical or nearly 

 vertical planes, there is every possible variety of direction in the 

 course taken by the water. This is a matter that cannot be even 

 guessed at Avithout a knowledge of the geology of the district. 

 With clays we must rank all such tenacious rocks as act in 

 a similar manner, and shut off communication. Thus, various 

 shales, slates, and marls are impermeable ; while others, even 

 if partially permeable, act as impermeable rocks, and divert the 

 course of underground waters. The former rank as clays, and 

 the latter as grits or limestones, according as the sandy or cal- 

 careous element preponderates. 



It is not only by their nature, composition, and texture 

 that rocks and certain strata shut off water ; in many countries 

 the rocks have been broken while being upheaved by mecha- 

 nical pressure from below ; and when broken, part of a stratum 

 has often been lifted up so as to occupy a position altogether 

 different from the rest. An absorbent rock may thus, by dis- 

 placement, be made to abut against a non-absorbent rock. A 

 crack may be opened in a series of rocks down which water may 

 be conveyed, or up which it may rise, according to the condition 

 of adjacent rocks. Communication may also be thus made with 

 the interior of the earth, and with places where some chemical 



