340 Geological Society. 



but its horizontal displacement is usually over 3000 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 Abereiddy and Aber- 

 castle (rembrokeshire).' By Arthur Hubert Cox, M.Sc, Ph.D., 

 F.G.S. 



The distnct is situated 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 investi- 

 gation, yet the stratigraphy and structure of the greater part is 

 now described for the lirst time. Abereiddy itself has been, since 

 the time of Hicks, a type-locality for the Llanvirn Beds, but 

 observations recently made by Prof. O. T. Jones showed that 

 the sequence required reinvestigation. It has now been found that 

 the Ordovician rocks of the disti-ict do not succeed one another in 

 a simple upward sequence, but that they have been thrown into 

 great folds and sometimes even overfolded. The folds have sub- 

 sequently been broken by extensive stril^e-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 Noi-th Pembrokeshire coast was pre- 

 viously quite unsuspected. 



There is a complete sequence of Ordovician rocks from near the base 

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

 Arenig rocks are a series of arenaceous strata (the Abercastle and 

 Porth Gain Beds) which correspond to the ' Nesnretus Beds ' of 

 Ramsey Island. These strata are in faulted relationship to the 

 Cambrian, so that the true base of the Arenig is not seen. The 

 arenaceous beds pass upwards without a break into Tetragraptus 

 Shales, which are in turn succeeded by the Bijtdus Beds. Llanvir- 

 nian volcanic rocks are represented in one pai-t of the district 

 by the Llanrian Volcanic Series, which begins high up in the zone 

 of Didymogrcqytns bi^diis, and in another part by the MttrcJiisoni 

 Ash, which forms the base of the D.-miirchisoni Zone. The Llan- 

 deilo Series compares closely with that of Carmarthenshire, and 

 does not contain any volcanic rocks as Avas at one time supposed. 



Contemporaneous igneous rocks occur at two horizons : — 

 (i) keratophp'es at a high horizon in the Tetragraptus Shales, 

 and (ii) quartz-keratophyres (soda-rhyolites) towards the top of 

 the D.-hrftdus Beds. The intrusive rocks (diabases) belong to 

 two types, («) subophitic quartz-diabases, and (b) ophitic diabases 

 without quartz. Both types were intnaded earlier than the main 

 folding, and consequently earlier than the cleavage and faulting. 



A great north-westerly line of distm-bance — the PwU Strodyr 

 Fault— cuts across all other structures, and brings on entirely 

 different groups of strata. 



