448 MM. Cole & Holland— Structure of Rhohett Faicr. 



pointed out that the mass of Ehobell Fawr was of composite 

 character, but was probably distinct from the series on Cader Idris. 

 The fragmental character of much of its material having been 

 recognized by the author of the Survey-Memoir, several theories are 

 discussed in that work to account for its stratigraphical position. 1 

 In spite of the lack of detail, when compared with other areas, with 

 which Rhobell Fawr has been represented on Sheet 75 S.E. of the 

 Map of the Geological Survey, the ashes, " breccias," and other 

 materials piled here upon the edges of the Lingula Flags are fully 

 dealt with in the text of the Memoir, and we are left finally to 

 select the most probable explanation of those put forward. Mr. 

 Clifton Ward, quoted on p. 59, suggested that this centre of 

 eruption broke out " after the close of the Tremadoc-slate period," 

 the material thrown out being slightly different in character to that 

 of the widely spread Arenig and Llandeilo series. Prof. Ramsay 

 (on p. 74) seems also to hold this view, suggesting in addition that 

 there is local unconformity between the Tremadoc and the Arenig 

 series, whereby the "Arenig" volcanic rocks of Rhobell Fawr come 

 to lie upon the denuded edges of the Lingula Flags. The remaining 

 view hinted at in the Memoir is that the volcano of Rhobell Fawr 

 is older than the Tremadoc slates. In the present paper, however, 

 at the risk of multiplying explanations, we hope to bring forward 

 evidence to show that it is in reality contemporaneous with the 

 Tremadoc. 



Mr. Clifton Ward, when mapping the Grit that has been regarded 

 as the base of the Arenig in this area, records the discovery of ''■ an 

 oblong, angular block of grit " a little below the summit of Rhobell 

 Fawr, " but no traces of the grit-bed were met with." ~ Whether 

 or no we lay stress upon the continuity of a lithological horizon 

 across a considerable area, there is no doubt that in this case gritty 

 beds occur only a mile away on Allt Lvvyd, above the Tremadoc 

 slates and below the Arenig volcanic series ; hence a similar occur- 

 rence on Rhobell Fawr, as Mr. Ward perceived, would have a real 

 and valuable bearing on the age of the underlying fragmental rocks. 



Rhobell Fawr appears as a broad rugged moorland indented on 

 the south-east side by a marked semicircular hollow, on the margin 

 of which the two most distinctive summits rise. The more northern 

 of these is Rhobell Fawr proper, reaching to a height of 2408 feet 

 above the sea-level ; the more southern is Moel Cors-y-garnedd, 

 about 1650 feet high. On the northern slopes of the volcanic mass 

 there are several picturesque little prominences, notably Rhobell-y- 

 Big, some 1580 feet in height. 



During a recent examination of the area, we have succeeded in 

 finding bands of quartzose grit both on the summit of Rhobell Fawr 

 and of Moel Cors-y-garnedd. In each case the bed is thin, measur- 

 ing about one foot at the former point and three and a half inches 

 at the latter. 



On Moel Cors-y-garnedd this grit forms a patch underlying the 



1 A. C. Ramsay, "The Geology of North Wales," 2nd edition (1881), pp. 58 

 and 71. 2 Eamsay, op. cit. p. 58. 



