122 D. C. Davies—Tke Millstone Grit of North Wales. 



fact that the chief difficulties met with in engineering operations 

 occurred beneath the river level. Here the chalk and marl are 

 thoroughly saturated, and, at only twenty feet below the level of the 

 water in the river, it was found necessary to close-timber the 

 top and sides of the tunnel before the brickwork could be put 

 in, on account of its giving way by the pressure of water. The 

 springs under the head of only twenty feet also boiled up with 

 great force. On the contrary, above the water level, drift- 

 ways were driven hundreds of feet in length, and left for months 

 together, without any timber shoring being required. It requires 

 little imagination to perceive how great will be the pressure of 

 water, and how immense the difficulties arising from it, in the pro- 

 posed Channel Tunnel, excavated beneath twenty or thirty fathoms of 

 water, and through a bed of hard chalk, doubtless abounding in 

 joints and fissures. It has been the low level, twenty feet beneath 

 that of the river, which has created the chief engineering difficulty 

 and expense attending the sewerage works at Norwich. 



VIII. — The Millstone Grit of the North-Wales Boedeb. 



By D. C. Davies, Oswestry. 



{^Concluded from the February Number, p. 73.^) 



The total thickness of Millstone Grit in North Wales does not 

 quite reach 300ft. This thickness is, I am aware, considerably 

 below that given by several authors, who notice this formation inci- 

 dentally ; but I am prepared to say, from actual and computed 

 •measurement, made on the spot, that the above figures are sub- 

 stantially correct. The Carboniferous system, from the base of the 

 Mountain Limestone to that of the Coal Measures in North Wales, 

 is but an epitome of the same beds in the North of England, the 

 middle member, the grits and sandstones, being thin in proportion. 



From whence were the materials for these beds derived ? This is a 

 difficult question to answer, and my reply must be taken rather as a 

 suggestion than as the solution of the problem. We must at once 

 pass over the limestone, because, from the absence of fragments of it 

 in the Grits, as well as from the fact that it is now itself partially 

 covered by them, we must suppose that it was entirely submerged 

 at the time of the deposition of the latter ; partly for the same 

 reasons we must, I think, pass over, for the most part, the closely 

 adjacent Silurian beds, although in these we should find all the 

 materials required. Quartz-veins for pebbles, felspar for cement, 

 iron and manganese for colour, and sand in abundance. These 

 materials we shall, however, find in greater abundance in the 

 Cambrian and other older rocks that make up now the western 

 promontories of Wales, some lying engulfed in Cardigan Bay, and 

 which once, in all probability, stretched almost indefinitely towards 

 that fabled Atlantis of the west which, to the geologist however, is 

 something more than a poet's dream. 



^ The " Pockets" (Figs. 4-7, p. 71) occur in one unbroken bed of sandstone, as 

 stated in the text, not in jointed and bedded rock, as represented in the woodcuts. 



