256 PROF. A. H. COX AND Ml!. A. K. WELLS ON THE [vol. lxxvi. 



1866. A. C. Ramsay.— ' The Geology of North Wales ' 1st ed. Mem. GeoL 

 Surv. 



Although he gives considerable space to the description of the rocks on 

 Cader Idris, Ramsay makes but brief references to the ground north thereof, 

 and these mainly concern the intrusive rocks. 



Ramsay himself favoured the view that the volcanic rocks of Cader Idris 

 were all of what was later termed 'Arenig' 1 age, though he acknowledged the 

 possibility of some of them being Tremadoc (p. 30). The intrusive rocks 

 were shown to be of two kinds : ' greenstone ' and ' felspathic porphyry,' both 

 kinds, although frequently transgressive, occurring in sill-like forms. 



Attention is called to the Dolgelley- Llyn Gwernan Fault, and to the re- 

 sultant repetition of both stratified and intrusive rocks. 



1867 & 1868. T. Belt.—' On the Lingula Flags of Dolgelley ' Geol. Mag. 

 1867, pp. 493 & 536 ; ibid. 1868, p. 5. 

 This classical paper gives an account of the lithology, palaeontology, and 

 general distribution of each of the three divisions of the Lingula-'Flug forma- 

 tions as developed in the Dolgelley district. Each division is further sub- 

 divided into Lower and Upper portions. Two or three fossil localities are 

 recorded in the area south of the Mawddach, but most of the fossils mentioned 

 were obtained in the district north of Dolgelley. 



1881. A. C. Ramsay. — 'The Geology of North Wales ' 2nd ed. Mem. Geol. 

 Surv. 

 In the second edition of the Survey Memoir, beyond certain changes in the 

 nomenclature of the groups, there is little additional to the first edition so far 

 as the geology of the Dolgelley district is concerned. An account is, however, 

 given of the grit-band which marks the base of the Arenig in the district east 



of Llwyngwril, but it is stated that ' from this point [that is, a point 



lj miles south-west of Arthog] all along the broken hills between the Maw- 

 ddach and Cader Idris its position is quite unknown.' 



1889. G. A. J. Cole & A. V. Jennings. — ' The Northern Slopes of Cader 

 Idris ' Q. J. G. S. vol. xlv, p. 422. 

 Petrological descriptions of some of the rocks on Cader Idris are given. 

 The felspathic porphyry of Llynau Crogenen was found to be a granophyre or 

 ' eurite ' similar to the acid intrusive rocks on Cader Idris itself . As regards 

 the stratigraphy, the authors stated that ' until those best qualified to judge 

 extend the local base of the Arenig to the northern front of Mynydd-y-Gader, 

 a hill north of the Cader-Idris escarpment, we must regard a large portion of 

 the eruptive series as of Upper or even of Lower Tremadoc age ' (p. 437). 



1896. P. Lake & S. H. Reynolds. — ' The Lingula Flags & Igneous Rocks 



of . . . Dolgelley ' Q. J. G. S. vol. lii, p. 511. 

 This paper contains a geological map of the district immediately south and 

 south-west of Dolgelley, on which the outcrops of the Ffestiniog, Dolgelley, 

 and Tremadoc Beds are separated. It was shown that on Mynydd-y-Gader 

 Tremadoc Beds yielding Dictyonema sociale were followed by a series of 

 rhyolitic rocks. An igneous band in the Dolgelley Beds was described as a. 

 lava, and was stated to be associated with ashy beds. 



1897. Sir Archibald Geikie. — ' Ancient Volcanoes of Great Britain.' 



On the evidence obtained by Prof. Cole & Mr. Jennings and by Mr. Lake 

 & Prof. Reynolds, and also as the result of his own observations, the author 

 gave it as his opinion that the volcanic activity started at least as early as 

 Tremadoc time (p. 177). 



1 Using the terms in the sense understood at the present day. No attempt 

 is made in this paper to trace the changes of nomenclature as applied to the 

 Upper Cambrian and to the Lower Ordovician rocks. The reader is referred 

 to Prof. W. G. Fearnsides's paper on ' The Tremadoc Slates of South-East 

 Carnarvonshire ' Q. J. G. S. vol. lxvi (1910) p. 144. 



