King — On the Jointing of Bocks in Relation to Engineering. 569 



position, and from 40 to 80 feet deep, which, from the arrangement of 

 their contents, have evidently been swallow holes, down which water 

 passed into subjacent catch basins. 



That similar openings are now in existence in the Channel maybe 

 considered as more than probable, seeing that '' sand pipes " in Kent, 

 as brought to light by Professor Prestwich, though at present at a 

 considerable elevation above the sea level, must have been at the bot- 

 tom o'' the sea during some portion of the Pliocene period, since occa- 

 sionally they contain the casts and other remains of marine shells 

 {Terelratula grandis and Lutraria elUptica) living at the time.^ 



But dislocations, fractures of disruption, inclined bedding partings, 

 porosity and sand pipes, are not the only sources of water leakage 

 that may be met with while excavating the Channel Tunnel : another, 

 if a not more serious one, remains to be noticed. Eock jointing possesses 

 characters favouring the idea of its being totally distinct from fractures 

 of disruption — rather, a divisional structure developed by no ordinary 

 mechanical agency; but whether this be the case or not, jointing re- 

 quires to be closely considered by alL parties immediately interested in 

 carrying out the proposed undertaking to a successful issue. 



The phenomenon now entered upon consists of regular and persist- 

 ently parallel fissures, characteristic of both stratified and unstratified 

 rocks : the softest shales and hardest granites are alike affected by it ; 

 and these may be of any geological age — from the Archseans up to the 

 Eocenes. Altogether independent of bedding or stratification, the 

 planes of jointing intersect those due to deposition, inasmuch as their 

 usual position is oblique, or rectangular, to bedding, whether it be 

 horizontal or inclined. Although, in general, from under an inch and 

 often many more apart (produced by the erosive action of the water 

 and other wasting agencies, also by stratic disturbances, making a joint 

 resemble an ordinary disruptive fracture), the conjunctive planes or 

 walls of a joint, in their normal or original condition, are in the closest 

 possible contact, appearing as if they had been made by the thinnest 

 and sharpest cutting instrument ; and this is equally the case with 

 foreign bodies, as blocks or pebbles of granite enclosed in conglome- 

 rates : the same planes have not uncommonly a surface as smooth, 

 and occasionally as lustrous (like those of mineral cleavage), as if they 

 had come direct from the hands of a marble polisher. The joints lie at 

 varying distances from one another, having usually one, two, or more 

 feet of separation ; but examples are rather frequent in which they 

 lie from an eighth to above an inch apart. 



In the case of rocks lying horizontally, jointing, besides affecting a 



2 Quarterly Journal of the Geol. Soc., vol. xiv., pp. 322-336. Being present at 

 tte meeting of the Geological Society (Jan. 21, 1857), when Prof. Prestwich's 

 Paper was read, I had an opportunity of examining the shells : the second one was 

 stated to he a Mya-like species. From certain characters it possessed, I felt certain, 

 as I there and then expressed, that it belonged to the still-existing species named 

 in the text. 



