Ixxii PEOCEEBIKGS OF THE GEOLOGICAL SOCIETY. [May 1 899, 



supply, which really amounts to a trade-circular, a well-known firm 

 state that they ' are prepared to send an experienced diviner to locate 

 water by the aid of the rod, and at the same time to give an idea 

 as to the probable depth at which water may be found and approxi- 

 mately the quantity available.' It is not clear whether the firm or 

 the diviner will give the particulars as to depth and quantity ; but 

 it is very consoling to be told, a little farther on, that ' it may, 

 however, truly be said that although the diviner may be advan- 

 tageously employed in discovering springs at a moderate depth, his 

 services can never replace, for deeper and more important supplies, 

 the carefully considered report of the experienced hydro-geologist.' 



I propose to glance at some of the special points which that 

 inferior creature, the experienced hydro-geologist, has to consider 

 carefully in this matter, and on the assumption that a large supply 

 is required, as, of course, for a small supply many points are of less 

 importance. 



The source must be some permeable formation of good thickness 

 and with a broad outcrop, as the quantity of water in any permeable 

 bed must depend on the amount of rain that falls upon it, and 

 this latter greatly on the area of surface exposed. A well, 

 therefore, must either be upon the formation that is to be the source 

 of supply or upon some overlying formation through which it can be 

 carried to the water-bearing stratum. These two classes of wells 

 sometimes differ greatly. 



In the first case, the well should be at a part towards which 

 underground water flows : away, therefore, from an escarpment or 

 ending-off of a formation, and towards the line of outcrop or where 

 the next overlying formation comes on. It should also be in low 

 ground, as a rule, so as to avoid needless depth. In the second 

 case, when a well has to be taken through some thickness of over- 

 lying beds to reach the water-bearing bed, different conditions 

 sometimes arise, unless the well is near the outcrop of the water- 

 bearing formation. 



The method of flow of water through the rocks must also be con- 

 sidered. In some, this is mostly through the pores or the spaces 

 between the particles of which the rock is built up ; but in some 

 water-bearing rocks very little passes in this way. Sometimes the 

 planes of bedding aff'ord a sort of channel, but at others these are 

 closed and well packed together. Often the flow is along joints, or 

 structural planes that have been formed after consolidation : fault- 

 planes may act in a like way. 



