68 Rainfall, Natural Drainage, 



in. — Subtekranean'Water-storage. 



^' That part of the rainfall which enters the earth, serves to 

 supply and renew the natural and artificial spring's, modify 

 the condition of strata, and to promote, or by its excess to 

 injure, vegetation. 



The proportion of rainfall which enters the earth will depend 

 not only on the nature of the soil and rock, the form of the ground, 

 and the total rainfall, but on the mode in which rain falls. 

 Where it falls in very heavy showers, there will be a large pro- 

 portion carried off along the surface. Where rain is light and 

 frequent, although the fall may be represented by a smaller 

 number of inches, a larger quantity will soak into the strata. 

 But although this is a general rule, it must also be remembered 

 that during long droughts the earth becomes cracked, and 

 that the cracks in time may become deep and then wide, and 

 capable of receiving an enormous proportion of the first 

 rains that fall. I have myself seen in the south of Spain 

 marly sandstones, otherwise not very permeable, cracked so 

 widely and deeply that they offered great impediments in getting 

 across the country. Clays also are not unusually cracked in 

 some parts of England to a depth of six or eight feet, or more, in 

 an ordinary summer ; and in hot countries gorges are thus 

 formed many yards deep, and so wide that a horse can hardly 

 be got across them. It is at any rate evident that in all places, 

 and under all circumstances, a large and important part of the 

 rainfall must sometimes enter the earth. It is also clear that, 

 Avhen there, it has no means of escape, except by subterranean 

 channels or by filtration through the solid rock. But such 

 filtration is not difficult. The peculiarly broken and cracked 

 condition of hard limestones and other brittle rocks renders them 

 capable of receiving very large quantities of water, while all the 

 softer limestones are eminently porous and absorbent even in 

 their solid mass. Hard rocks are always fissured, and often 

 cavernous. Limestones abound with extensive open spaces, and 

 in some cases these contain lakes ; in others large rivers run 

 through them. The great caverns of Adelsberg in Carinthia, and 

 Kentucky in North America, are examples of this ; and many 

 others nearer home, on a smaller scale, as in Derbyshire, York- 

 shire, Somersetshire, and South Wales, will suggest themselves to 

 every reader. But sandstones of the harder kind are also fissured ; 

 and granites invariably abound with joints, leading occasionally 

 to open spaces, so that Avater can be reached and obtained from 

 them by the ordinary operations of well-sinking. Mining- deve- 

 lopes very clearly the peculiarities of all these rocks, and exhibits 



