igo THE BOOK OF NATURE STUDY 



the grains of sand making up the original sandstone rock. Other 

 rocks, the basalts and granites, for example, under the influence 

 of water, simply grow rotten, like an old exposed piece of iron 

 covered with layer after layer of rust ; they pass by insensible 

 gradations into clay, whereas limestones keep a firm surface but 

 seem to have largely melted away into thin layers of sticky clay. 



Here, then, are two of the agents making soil out of rock 

 roots to burst, water to rot and dissolve ; the work of a third 

 great agent frost will be most in evidence if an old face of the 

 quarry be examined just after a thaw. Failing that a visit may 

 be paid to an old brick or stone wall, especially one backing against 

 a bank that will keep plenty of moisture in the wall. After the 

 frost has departed the ground at the foot of the quarry face or the 

 wall will be covered with fragments of stone or brick which have 

 obviously only just fallen away from the clean broken faces above. 

 It is easy to show, by tightly tying in the cork of a bottle filled 

 with water and exposing it in a frosty night, that water expands 

 considerably in the act of turning into ice ; and further, that it 

 exerts a pressure on whatever resists this expansion such as very 

 few materials can withstand. The stones or bricks of the wall, 

 and even the stones a little below the surface of the ground, are 

 generally saturated with water; they become rent open as the 

 water expands on freezing and fall in pieces as it thaws again. 

 Thus the expansion of water on freezing must be added to its dis- 

 solving and rotting powers as one of the agencies reducing rocks 

 to soil, although, since frost in this country rarely penetrates the 

 ground to a greater depth than a foot or eighteen inches, it is in 

 the upper layers, the soil, that its disintegrating action is most felt. 

 These agencies roots, water, and ice may at first sight appear too 

 slow and trivial to have been capable of forming four or five feet of 

 soil and perhaps ten or twelve feet of rotten stone, but it is only 

 necessary to look at an old castle or unrestored church to realise 

 how active the " tooth of time " can be. Five hundred years 

 carves into the most fantastic shapes the face of even the hardest 

 building stones, stones which have been kept comparatively dry by 

 exposure instead of being buried in the wet ground, where also 

 roots are at work. Yet many periods of five hundred years have 

 elapsed since our present layer of soil began to be made, since the 



