Belt— On the '' Lingula Flags" 493 



rivers, ice, springs, damp, and frost are powerless to wear away 

 rocks and to cut out escarpments valleys and rock-basins ; and 

 secondly, that tke sea can do and does such work. This, no light 

 task truly, must be done, if it can be done, not by mere assertions 

 of individual opinion, or mere statements based on hasty and pre- 

 judiced observations, but by hard work and sound reasoning. Not 

 with us, but with our opponents, lies the onus probandi. 



Errors in the First Part of this Paper. 



Page 451, line 16 from bottom, for " action " read " actions." 

 Page 451, line 11 from bottom, for " Portland Stone in part of the Isle of Purbeck" 

 read " Purbeck and Portland Beds in Dorsetshire." 



Page 453, line 15 from bottom, after "follow " insert " us." 



III. — On the "Lingula Flags," ok " Festiniog Group" 



OF THE DOLGELLY DISTRICT. 



By Thomas Belt, F.G.S. 



[PART I.] 



THE strata lying above and below the Lingula Flags have already 

 been well described and illustrated : the Menevian Group below, 

 by Messrs. Salter and Hicks, and the Tremadoc Group above, by 

 Messrs. Homfray, Ash, and Salter. The great mass of strata lying 

 between has not fared so well, though several notices of it, to which 

 I shall refer, have appeared. In the present paper I propose to 

 describe these strata in detail; and the remarks I have to offer 

 embody the results of three years' researches, during part of which 

 I have had the advantage of the company and able co-operation of 

 Messrs. Ezekiel Williamson and J. C. Barlow, whose discoveries 

 I shall have to mention in my description of the rocks and their fossil 

 contents. To facilitate the study of the district around Dolgelly, 

 which is exceedingly faulted and complicated, I have carefully 

 mapped out nearly the whole of the rock exposures. This may 

 seem to have been unnecessary, seeing that we have already the 

 Geological Survey maps of the district. But since the officers of 

 the Survey examined and mapped out the rocks of Merionethshire, 

 from fifteen to eighteen years have elapsed, and the maps which 

 then added so much to our knowledge are now far behind our 

 requirements. The whole of the strata lying between the Tarannon 

 shale and the Cambrian grits are there coloured alike. Neither the 

 Arenig nor the Tremadoc rocks are recognised ; and we now know 

 that the strata there named "Lingula Flags" include at least three 

 distinct and diverse groups. 



In 1847 Pi'ofessor Sedgwick separated the Tremadoc rocks from 

 the " Lingula Flags," calling the latter the Festiniog Group. Since 

 then Mr. Salter has been the pioneer in their investigation. His 

 discovery in 1863 of Paradoxides Davidis in the slates of St. David's 

 gave an impulse to the study of these old rocks, that has resulted in a 

 rich harvest of Primordial trilobites, chiefly through the indefatigable 



