74 Rainfall^ Natural Drainage^ 



when no change of mineral species is effected, there is frequently 

 an atomic change in rocks, such as is shown in the rearrangement 

 of the particles from mechanical aggregation to crystalline texture. 

 For this water is necessary. The soft clay, moulded on some 

 organic body — a cockle-shell or the fragment of a bone in an 

 altered state — is thus found to afford important evidence of the 

 condition of the earth's interior, and the movements that have 

 taken place there. 



But it may be desirable to explain a little more fully the 

 law of nature in reference to springs of water. The ordinary 

 arrangement of rocks is that of stratification. They have been 

 originally deposited horizontally with and from water, but they 

 have since undergone great change. The water in fact has been 

 partially got rid of, and the mud consolidated. They have also 

 been thrust up from being below the sea to a position sometimes 

 many thousands of feet above that level, and in the elevation 

 some portions have been broken, and very large quantities have 

 been pared away by water action. Thus limestones have become 

 cavernous, sandstones are full of crevices, and the whole mass 

 has been shut off into boxes, having very slow communication 

 one with another. Thus, also, water entering a second time and 

 from above, sinks down, penetrating every crevice, occupying 

 every cavity, carried on from one box to another, or filling one 

 before passing on to the next, running down hill whenever the 

 strata admit it, often forced up hill when there is pressure behind 

 and there are no other means of escape ; and, in a word, circu- 

 lating among and through the strata, and the faults and joints 

 produced in them, and while simply obeying its own laws, 

 conveying the means of chemical change from one part of the 

 earth to every other part, and from the surface to the greatest 

 depth to which strata reach. In all strata there is at some 

 depth, great or small, a surface of absolute saturation. If this 

 surface be reached by a well or boring, or if it be intersected by 

 a natural cliff or hillside, or by an artificial cutting, the water 

 will escape, or can be brought to the surface by pumping. To 

 this depth the rock will always absorb. Below it water will be 

 yielded up. But it may, and often does happen, that long before 

 reaching the depth below which the whole rock is saturated, 

 there are extensive sheets of water kept back by impermeable 

 strata. These also, when reached or intersected, yield water, 

 but if penetrated, the water would pass downwards to the rocks 

 below, and the wet rock become drained. There are thus sur- 

 faces of partial saturation. By sinking to or below these surfaces 

 extensive and important results have been effected, both in well- 

 sinking and drainage. 



It is easy to see that two very different conditions of the water 



