ARTESIAN WELLS IN DORSET AND ELSEWHERE. 207 



Chalk is fully protected by a considerable thickness of Tertiary 

 beds. 



It is beyond my purpose even to speculate on the possible 

 causes for this remarkable failure in a distant locality ; but a few 

 remarks on the variation in the underground waters of the 

 Chalk, so far as they are known in the county of Dorset, will not 

 be out of place, and may be of some practical utility. It is 

 probable that where the Chalk in this county is covered by 

 Tertiaries which have protected it from more recent denudation, 

 the beds of Chalk first met with in boring operations are those 

 of the upper part of the series. Hence it follows that both at 

 Wimborne, at Bovington, and at Spyway, at all which places the 

 Chalk surface is protected by Tertiary beds to a greater or less 

 extent, the Chalk first encountered by the boring-tool is mainly 

 on the same horizon in each case ; yet the results as regards 

 water supply are very different. We have in the first instance, 

 at Wimborne, abundance of sub-surface water and full artesian 

 pressure in the very top of the Chalk ; and this, moreover, is 

 enormously increased by merely going about 100 feet deeper. 

 Here, therefore, the highest part of the Chalk is full of water 

 under strong hydrostatic pressure, whilst at Bovington more 

 than 100 feet of Chalk had to be pierced before the first spring 

 was reached, and nearly 300 feet before an adequate supply was 

 obtained ; and in this case the pressure is only sub-artesian, 

 i.e., the water does not quite reach the surface. 



It is evident, therefore, that position with regard to existing 

 contours is an important factor in an artesian well, and the 

 valley of the Allen at Wimborne seems to fulfil the requirements 

 of the case in a remarkable degree. But there is also another 

 element in the problem, viz., the character of the Chalk 

 encountered during operations. It has been said that perme- 

 ability in the Chalk depends not so much on the nature of the 

 Chalk itself as on the fissures by which it is traversed. This 

 is well illustrated by the experience of the headings in the 

 Bournemouth Waterworks at Wimborne. At the same time it is 

 not incorrect, in a general sense, to regard the Chalk formation, 



