570 Proceedings of the Royal Irish Academy. 



more or less vertical position, is found to be resolvable into two or 

 more directional series or systems, each, being distinguisbed by uniform 

 parallelism, also by a definite and an independent course, of its mem- 

 bers. It is also found that these systems are traceable, as in the lime- 

 stone districts of the west of Ireland, over areas hundreds of miles in 

 extent ; often, however, more developed in some places than in others, 

 or suddenly disappearing here, and as suddenly setting in with great 

 force elsewhere. The remarkable approximate parallelism of these 

 systems with the meridians and the equator seems to give propriety 

 to one being named meridional and the other equatorial. 



That Jointing ought to command the closest attention of engineers 

 engaged on subaqueous works must now be evident. The following 

 case is added, however, to make the statement still more obvious : — 



During the famine period of 1845-1848 in Ireland, the Board of 

 Public Works commenced the construction of a canal through beds of 

 Carboniferous limestone for drainage purposes, and the opening out of a 

 water communication between Loughs Corrib and Mask of about four 

 miles in length. Eut, on nearly completing the work, it was found 

 that the joints, well developed in the limestone, and probably taken 

 to be little more than superficial, carried off all the water, necessi- 

 tating much additional and unexpected outlay to remedy the defect. 

 Thus after an expenditure, as I am credibly informed, of £40,000 of 

 public money, what was intended to be a canal turned out to be 

 nothing else than a dry ditch ; and as such it still remains — a warning 

 to all engineers not to neglect becoming acquainted with an important 

 geological phenomenon. 



Passing to the Channel Tunnel, it is true that this scheme has 

 nothing to do with rocks in which jointing is so well developed as in 

 the Carboniferous limestone and other Primaries. Still, with the facts 

 that have been before us, together with others which are yet to be 

 noticed, I feel confident the undertaking, if it ever be properly 

 entered upon, will develop graver difficulties than any that have been 

 conceived by those who are actively promoting it. 



For anything known to the contrary, it may be safely assumed 

 that the rocks to be penetrated by the tunnel are the sandstones, 

 shales, and chalk formations, included in the Cretaceous andNeocomian 

 Systems, known to exist in the counties that have been mentioned, 

 also in the Bas Boulonnais on the opposite seaboard of Prance, 



As stated before, these rocks in Kent and Surrey have been thrown 

 into flexures, running axially in a nearly east and west direction ; while 

 parallel with them is a number of dislocations or faults, and fractures. 

 A marked feature of the flexures is, that they are broken right across 

 by transverse gorges, seen particularly where they are in the form of 

 ridges, through which the main rivers flow into the Thames and its 

 estuary on the north, and into the Channel on the south. 



Different hypotheses have been offered in explanation of these east 

 to west valleys and north to south gorges : the generally accepted one 

 ascribes them to the mechanical forces which upheaved the wealden 



