94 Reports & Proceedings — Geological Society of London, 



The distribution and character of these beds are described. The 

 ' Mottled Beds ' form the base of the Silurian and rest sharply on the 

 underlying beds, and there is evidence of complete discontinuity at 

 this level; they have proved of great service in elucidating the 

 structure. The Ifonograpttis spp. beds contain graptolites which 

 elsewhere pertain to the zones of Monograptus triangulatus, M. cyphus, 

 and If. acinaces ; but another zone, that of M. atavus, has not been 

 proved, although it probably occurs. The Derwen group consists 

 of a regular alternation of muH stones and shale-bands with graptolites, 

 which have also proved of service in mapping. Only a small thickness 

 of the Ystwyth stage occurs, and no subdivisions are attempted. 



The rocks are sharply folded, and sometimes overfolded, towards 

 the east. Their axes range approximately north-north-east and 

 south-south-west; the folds in the central area pitch northwards, 

 but north of the Dovey a southerly pitch sets in. Each large fold is 

 composed of a number of smaller folds having parallel axes, and 

 changing in pitch more frequently than in the larger folds. Strike- 

 faults of considerable magnitude range nearly parallel with the folding 

 axes, and are in all cases overthrusts towards the east. 



Of greater interest are the transverse faults ranging nearly east- 

 north-east and west-south-west. Most of these are small, but their 

 course across the higher ground is indicated by well-defined notches 

 in the ridges that they cross. Two of these faults, the Pennal and 

 Llyfnant Faults, are shatter-belts. The Llyfnant Fault displaces 

 several folding axes, and overthrusts to the east on the north side. 

 Its vertical displacement is on an average about 300 feet, but its 

 horizontal displacement is usually over 3,000 feet. It may therefore 

 be called a 'tear-fault'. Both the Llyfnant and the Pennal Faults 

 exercise some influence upon the drainage system of the area. 



A brief comparison of the succession with other districts is added. 



2. " The Geology of the District between Abereiddv and Abercastle 

 (Pembrokeshire)." By Arthur Hubert Cox, M.Sc, Ph.D., F.G.S. 



The district is situate north-east of the area occupied by the 

 pre-Cambrian rocks of St. Davids, and it is bounded on its northern 

 side by the Pembrokeshire coast. Although some parts of this 

 district have already been the subject of geological investigation, yet 

 the stratigraphy and structure of the greater part is now described 

 for the first time. Abereiddy itself has been, since the time of 

 Hicks, a type locality for the Llanvirn Beds, but observations 

 recently made by Professor O. T. Jones showed that the sequence 

 required reinvestigation. It has now been found that the Ordovician 

 rocks of the district do not succeed one another in a simple upward 

 sequence, but that they have been thrown into great folds and some- 

 times even overfolded. The folds have subsequently been broken by 

 extensive strike-faulting. The limbs of the folds increase in steepness 

 as the pre-Cambrian massif is approached. This folding brings up 

 strips of Cambrian rocks, the presence of which on the North 

 Pembrokeshire coast was previously quite unsuspected. 



Thei'e is a complete sequence of Ordovician rocks from near the base 

 of the Arenig Series to high up in the Grlenkiln Group. The lowest 



