412 REPORT — 1882, 



anything, it is that a tunnel driven in the lowest beds of chalk must come 

 very near a bed from which large springs are discharged on land, and that 

 it will probably have to pass into this bed. I may here remark that it is 

 a fallacy to suppose that if a heading were driven across the Channel in 

 one bed, without meeting with water, it would prove that a tunnel could 

 be driven on the same line equally free from water. The vertical depth 

 of a heading is only 7 feet, that of the excavation for a tunnel would be 

 at least four times that depth. In the Severn Tunnel works, headings 

 have been driven in perfectly dry strata, yet, when the same have been 

 enlarged for the full-sized tunnel, large quantities of water have been met 

 with in the adjoining strata. 



It has not only been asserted that very little or no water will be met 

 with in the lowest beds of chalk, but also that so much will be met with 

 in the higher beds as to make it impracticable to tunnel through them. 

 Large numbers of wells have been sunk all over the country, and other 

 excavations have been made at all depths and in all parts of the chalk 

 formation for the purpose of obtaining water, and our knowledge of the 

 mode in which water occurs in that formation is fairly complete. Much 

 has been written on the subject. Professor Prestwich treats of it in his 

 work on the water-bearing strata of London,' and gives references to 

 earlier writers, but much information has been gained on the subject since 

 the date of Professor Prestwich's work. In the ' Proceedings of the 

 Institution of Civil Engineers,' from the year 1839 down to the present 

 time, numerous papers on the water-bearing qualities of the chalk may be 

 found, and in the discussions which followed them the most eminent 

 geologists of the day, Dr. Mantell, Dr. Buckland, Professor Ansted, and 

 others, took part. 



The following are, I believe, well-established facts on the subject in 

 question. 



Solid chalk absorbs a large amount of water, but it parts with it with 

 extreme slowness, so that it cannot be said to be truly permeable. It is 

 only from the fissures which traverse the chalk formation, and from the 

 cavities found along its planes of stratification, that water can be obtained. 

 At a variable depth below the surface of the land the body of the chalk is 

 saturated, and such fissures and cavities as may occur there are full of 

 water, not stationary, but slowly flowing from the higher ground inland, 

 towards the I'iver valleys and sea-shore, where the water can find an 

 outlet. Thus, there is a circulation of water underground, just as there is 

 above ground on other less porous formations. Moreover, as the streams 

 and watercourses on the surface are independent of one another, so are the 

 underground channels in the chalk. If, in excavating chalk, a fissure is 

 cnt across, a certain supply of water may be obtained ; if a second fissure 

 is cut through, a further supply may be obtained ; and genei-ally it will 

 be found that if the water is pumped from one fissure, it does not imme- 

 diately diminish the flow from adjoining fissures ; and, in like manner, if 

 the flow of water from one fissure is checked, it does not necessarily 

 increase the flow from those adjoining. In course of time, lowering the 

 water-level at any fissure will affect the supply to adjoining fissures, inas- 

 much as all draw their supply from the same saturated mass of chalk ; 

 but as a consequence of the extreme slowness with which chalk gives np 



' A Geolof/ical Enqiiirii respecting the Water-hearing Strata round London, by 

 Joseph Prestwich, Junior, F.G.S., London, 1851. 



