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Vol. 66.~\ TREMADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 167 



which he identified as of Arenig age, the 4000 feet for the Upper 

 Arenig and Llandeilo rocks seemed a good deal ; and when 

 Hopkinson & Lapworth, 1 and Lapworth, 2 had further shown that 

 these graptolites belong to a horizon which is higher than the 

 Llandeilo limestone of South Wales, the acceptance of 4000 feet 

 for the thickness of Upper Llandeilo rocks became impossible. 

 There must be repetition, and, as there is no evidence of isoclinal 

 folding, that repetition must be accomplished by faulting. 



The Penmorfa Fault. 



Like most great faults, the Penmorfa Fault is a series of parallel 

 shatterings, which have a common direction. It separates forma- 

 tions as far apart as Tremadoc and Upper Llandeilo ; but, since 

 it brings slate against slate along the strike, its exact position is, in 

 the absence of fossils, very difficult to localize. 



The eastern course of the fault as it enters the district here 

 described is concealed by the Glaslyn alluvium. It must pass 

 between Yyns-towyn and Tnys-galch, probably by way of the 

 Cambrian Pailw ay-station. It ranges through the Ynys on which 

 Pen-syflog House is built, and brings the Garth Hill Beds with 

 Angelina there, within 50 yards of Llandeilo slates containing 

 Climacograptus sclidrenbergi. 



Across the Morfa to Glan-morfa it lies beneath recent sand and 

 silt ; but, along the south-western edge of the mound Hen-fynwent 

 (the site of the Roman villa), a little cliff on the field side shows a 

 fine section of an overdrag affecting Ordovician slates. In Penmorfa 

 village the road-cutting at the corner between Capel Seion and 

 Plas-isaf shows Garth Hill Beds and splintered Ordovician rock side 

 by side. The new well-house is on the Ordovician side of the fault, 

 but Tremadoc fossils can be found 50 yards to the west of it. Above 

 Tyddyn-dicwm the fault continues obliquely up the hill, and passes 

 the 600- foot contour before it comes down, and is buried beneath 

 the Cwm Ystradllyn Drift. Some 2 miles farther along, a fault of 

 similar hade and direction can be proved along the crags of 

 Tyddyn, south of the river, at Dolbenmaen. 



Along this line the fault maintains an even course, the geometry 

 and general outcrop of which indicate a strike of about N. 50° W. 

 to N. 60° W. and a general gentle dip to the north of north-east. 

 In order the better to study the fault-plane, I arranged with 

 Mr. Greaves (Wern), the owner, and Mr. Jones, the farmer, to 

 have trenches cut across the fault, at the locality, above Tyddyn- 

 dicwm, where its course is most open, and where the Tremadoc 

 and Llandeilo fossils had been proved in closest apposition. The 

 spots chosen are in the big field containing the westernmost 

 of the Tyddyn-dicwm iron-ore trial-holes, and are both above the 

 600-foot contour, on the southern slopes of Craig-y-gesail. The 

 sections are dip- sections and are best described by true-scale 

 diagrams (see figs. 1 & 2, pp. 168-69). 



1 Quart. Journ. Geol. Soc. vol. xxxi (1875) p. 636. 



2 Ann. & Mag. Nat. Hist. ser. 5, vol. iv (1879) p. 340. 



