Vol. 52.] LINGULA-FLAGS AND IGNEOUS ROCKS OF DOLGELLY. 511 



30. The Lingula-Flags and Igneous Rocks of the Neighbourhood 

 of Dolgelly. By Philip Lake, Esq., M.A., F.G.S., and S. H. 

 Reynolds, Esq., M.A., F.G.S. (Read May 27th, 1896.) 



[Plate XXIV.— Map.] 



Contents. 



Page 



I. Introduction 511 



II. Description of the Area 512 



III. Geological Structure , 512 



IV. The Stratified Rocks 513 



(a) The Slopes of Mynydd Grader. 



(b) West of the Dolgelly Fault. 



(c) Upper Volcanic Series. 



V. The Intrusive Diabases and their Relations to the 



Stratified Deposits 517 



VI. Age of the Intrusive Rocks and of the Faults 521 



VII. Conclusions 521 



I. Introduction. 



The magnificent section displayed in the precipitous northern face 

 of Cader Idris has attracted the attention of many geologists, and 

 has proved so seductive that the lower-lying country to the north 

 has been almost neglected. It is with the latter that we are 

 concerned. Here the Lingula-Flags are well developed, and are 

 associated with contemporaneous volcanic rocks and with remark- 

 able intrusive masses of diabase. 



But little has yet been published concerning these rocks. Belt l 

 in his classical paper upon the Lingula-Flags has dealt with the 

 general sequence in the district, and refers to several localities in 

 the neighbourhood of Dolgelly. Cole aud Jennings 2 have examined 

 the rocks immediately south of the area covered by our map, and 

 they were led to believe that the volcanic eruptions of this region 

 began at least as early as the Tremadoc period ; but, as they remark, 

 the point requires further investigation. We are able to justify 

 their belief, for we have discovered a well-defined band of andesitic 

 lava in the Lingula-Flags themselves. 3 



Several references to the region will also be found in the 

 Geological Survey Memoir on North Wales 4 ; and the account of 

 the Lingula-Flags in the Palaeontological Appendix is in this 

 respect of particular interest. 



1 Belt, 'On the " Lingula-Fla,%s " or "Ffestiniog Group" of the Dolgelly 

 District,' Geol. Mag. 1867, pp. 493-495, 536-543; ibid. 1868, pp. 5-11 & pi. ii. 



2 Cole & Jennings, ' The Northern Slopes of Cader Idris,' Quart. Journ. 

 <J eol. Soc. vol. xlv. (1889) pp. 422-439. 



3 [It should be mentioned that Messrs. Salter and Hicks have already 

 recorded contemporaneous trap from the Lingula-Flags (Quart. Journ. Geol. 

 Soc. vol. xx. 1864, p. 241) ; and that Dr. Hicks finds volcanic material in the 

 Menevian and even at the boundary between the Caerfai aud Solva beds (Geol. 

 Mag. 1894, p. 405).— July, 1896.] 



4 Ramsay, ' The Geology of North Wales,' Mem. Geol. Surv. vol. iii. 2nd ed. 

 1881. 



