part 3] LOWER PALAEOZOIC OF AltTHOG— DOLGELLEY. 255 



hollow situate on the outcrop of a big granophyre-sill. On 

 account of the prevailing high dip (about 40°), dip-slopes are 

 frequently almost as steep as scarp-slopes. The regularity of the 

 various scarps is considerabty modified by the presence of numerous 

 intrusive masses, by jointing and faulting, and also by the effects 

 of rejuvenation of the drainage-system. Accordingly, the features 

 appear very diversified and difficult to explain, until the geological 

 structure has been worked out in detail. 



The entire range is ,cut through by the deep gorge-like valley of 

 the Afon Grwynant, and by the broader and shallower valley of the 

 Afon Arthog, and the main drainage is accomplished by these two 

 streams, especially by the former of the two. This stream has 

 intersected the Gwernan Fault- valley in such a way that it now 

 receives tributaries from both halves of that valle} r . It also 

 Teceives numerous minor tributaries that flow as subsequent 

 streams along small hollows on the outcrops of softer beds. The 

 rejuvenation which is so marked in the main stream has not 

 greatly affected the smaller tributaries, and as a consequence their 

 valleys tend to hang above the main valley. Rejuvenation has 

 not proceeded so far up the Afon Arthog, and therefore only the 

 lower course of this stream runs through a gorge. 



The hill-slopes facing the estuary and the lower slopes of the 

 •Gwynant and Arthog valleys are (or were) densely wooded. Else- 

 where much of the ground is extremely rough and craggy, and 

 remains entirely uncultivated. The more open ground is mainly 

 used as a grazing ground for sheep and cattle, only very small 

 patches above the wooded level in the Grwynant and Arthog valleys 

 being devoted to tillage. The scenery is accordingly varied, the 

 estuary of the Mawddach and the district around Dolgelley being 

 numbered among the most picturesque areas of North Wales, and 

 •exposures are everywhere so numerous that there is no need to 

 indicate individual exposures on the map. 



(b) History of Previous Research. 



The geology of this strip of countiy is intimately related to that 

 of the neighbouring Cader-Idris range. But, although Cader Idris 

 itself has attracted the attention of several eminent geologists, 

 little detailed work has been devoted to the foot-hill district on 

 the north. 



1847. A.Sedgwick. — 'On the Classification of the Fossiliferous Slates of 

 North Wales ' Q. J. G. S. vol. iii, p. 147. 

 The presence of the Ffestiniog [that is, the Upper Cambrian] Group beneath 

 the contemporaneous porphyries of the Cader-Idris range is established. 



1850. Publication of the Geological Survey 1-inch Map. Quarter- Sheet 



59 N.E. Surveyed by A. C. Ramsay & A. E. Selwyn. (Revised 



1855.) 



The intrusive rocks are separated, and the transgressive character of the 



Crogenen porphyry is clearly displayed by the manner in which it encroaches 



upon a band of ' ashes ', the Lower Basic Series. No boundary is drawn, 



however, between the Upper Cambrian and the Ordovician rocks, all the 



strata below the above-mentioned ' ashes ' being indicated by a single colour. 



