and Subterranean Water Storage. 69 



tliem broken and cracked, and allowing water either to pass 

 through them out of sight, or collect into pools and open spaces 

 in the interior of the earth. 



It cannot be necessary to point out that the effect of a thorough 

 system of artificial drainage in a large district must be to carry 

 the rainfall beneath the surface both inore rapidly and more 

 completely than would be the case otherwise — more rapidly 

 because it is conveyed at once into straight watercourses, avoid- 

 ing friction, and shortening distances ; and more completely 

 because there is time allowed for considerable quantities to 

 penetrate'far down in places where the natural absorbing surfaces 

 of rock would act slowly. This is especially the case where the 

 surface of a rock is covered by a thick coating of vegetable soil, 

 because it must then be very liable to become choked. 



It requires only a very superficial examination of rocks to discover 

 how fit they are to absorb water, and how it is that they have 

 become so fit.* All rocks, without exception, that have been 

 long exposed to the air have become weathered, and a part of 

 this weathering consists in the production of innumerable cracks 

 on the exposed surface. In very hard rocks these are often 

 dependent on the presence of particular minerals or of veins of 

 foreign material ; but in the hardest granite there is always some 

 way in which moisture can enter, and once there its alternate 

 expansion and contraction due to changing temperature, espe- 

 cially near the point of greatest density of water (about 39^ F.), 

 is sure to destroy the texture very rapidly. A few examples of 

 the observed influence of weather on rocks of different kinds 

 may be useful. I will give only some results of personal inves- 

 tigation in various parts of the world. 



Of all rocks it might be supposed that the granites were those 

 most able to resist weathering ; and no doubt some varieties 

 do resist atmospheric action to a very great extent ; but 

 all rocks of this kind consist of alternate veins of harder and 

 softer material. Of some the harder is the largest in quantity. 

 Such rocks are valuable for building, and are little affected 

 by weather ; but even in such hard granites there is always 

 a certain amount of change produced in time — the surface 

 becomes rough, some of the crystals are decomposed, and 

 plants find room for their small rootlets in the crevices thus 

 formed. Many of the softer varieties show the results of 

 weather much more clearly. In a quarry one inay often find 

 that the recently cut face of the stone is discoloured to a depth 



* It must be remembered that all rocks withiu the earth were originally depo- 

 sited with, and under water, aud have never since been deprived of this water 

 completely. Throughout all changes that may have taken place in their compo- 

 sition water has always been present. 



