ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixiii 



northern part of the Merioneth antichnal. The Lingulee beds are 

 agahi found on the eastern flank of this antichnal, over which, 

 therefore, their surface must be conceived to pass as an imaginary 

 surface rising possibly to a great elevation above the lovirer beds 

 which rise to the existing terrestrial surface along the Merioneth 

 anticlinal, and which appear to be some of the lowest beds recognized 

 in the whole series. Our imaginary surface will descend to meet 

 again the existing terrestrial surface on the eastern flank of the 

 anticlinal along a line parallel to the anticlinal axis ; after which, 

 dipping to the S.E., it meets an enormous fault running along the 

 valley between Dolgelly and Bala, the downcast of which on the N.W. 

 side, near Arran Mowddy, is estimated at not less than 12,000 feet. 

 By this fault the Lingulee beds are again brought to the surface, be- 

 neath which they again dip very rapidly in the same south-easterly 

 direction as before, to descend beneath the Berwyns to an enormous 

 depth. It is however in the north-eastern corner of this district, in 

 Denbighshire, that our stratum, or its equivalent, must descend to its 

 lowest level. It must there form a depression in which is accumulated 

 the whole series, not only of the Lower Silurian beds above the one 

 in question, but also of the Upper Silurians found in that locality. 

 To the S.E. of Cader Idris the Lingulse beds pass under the Lower 

 Silurians of S. Wales. 



In contemplating the external features of a mountainous tract, 

 the eye never fails to exaggerate greatly the real relations which 

 the vertical elevations bear to the horizontal distances, but it will 

 frequently fail to give us an adequate conception of the geological 

 elevations and depressions of the same tract, as they would be 

 presented by the surface of a continuous stratum, if it were cleared 

 of its superincumbent mass where covered up, and restored where 

 it has been destroyed by denuding agencies. Professor Sedgwick 

 estimates the thickness* from the bottom of the Lingulse beds to 

 the top of the Bala group at something more than 16,000 feet. 

 Mr. Jukes estimates the thickness at 24,000 feetf, but this esti- 

 mate may perhaps apply more accurately to localities which present 

 a full development of the lower or Trappean group of this mass. 

 Now about the valley of the Dee, and north of the Berwyns, the 

 Bala beds dip directly under the Upper Silurians of Denbighshire, 

 at a level not very much above that of the sea ; and consequently 

 the depth of the equivalent of the Lingulse bed is probably there 

 at the depth of at least 16,000 feet, taking the lower of the above 

 estimates. Again, if we conceive the Lingulse beds continued across 

 the Merioneth anticlinal, they would probably rise to the height of 

 4000 feet above the level of the sea. lience between the summit 

 of this geological range and the bottom of the geological depression 

 of Denbighshire, the difference of level may be estimated at about 

 20,000 feet, or nearly four miles. 



We have already seen that the basset edges of the Linguloe bed, 

 or its equivalent, near Tremadoc and on the shores of the Menai, are 



* Quart. Jouin. Geol. Soc. vol. viii. p. 147. 

 t Quart. Journ. Geol, Soc. vol. iv. p. 300. 



