﻿Vol. 66.~\ TKE3IAD0C SLATES OF SOUTH-EAST CAEXAEVONSHIKE. 185 



the thrust-planes, large laccolites of gabbroid dolerites have been 

 intruded at a date subsequent to both cleavage and faulting, and a 

 metasomatic development of pisolitic iron-ore has taken place along 

 the line of the most notable (Penmorfa) thrust. Glacial accumu- 

 lations mask the geology of the north-western district, but the 

 adaptation of surface-features to underground geology is very 

 perfect, and the exposures of the outcrops in the driftless area of 

 the south-east are very complete. There is a considerable develop- 

 ment of post-Glacial accumulations at or near sea-level. 



The stratigraphical succession begins with the Maentwrog Beds 

 and ends somewhere in the Upper Llandeilo or Caradoc. As far 

 as the unconformity which transgresses the Tremadoc Slates the 

 succession is simple and complete; but the Arenig rocks are not 

 well exposed, and the Llandeilo and later rocks have been involved 

 in much earth-movement. 



The Maentwrog Beds are dark rusty-grey slates with ringers 

 of felspathic material, and are like the Lower rather than the 

 Upper Maentwrog Beds described by Belt at Llanelltyd. 



The Ffestiniog Beds are a thick series of grey flags and 

 compact grauwackes, with the Lingulella Band at their top. 



The Dolgelly Beds are black slates, as at Arenig, but at the- 

 base of the Peltura horizon contain thin felspathic bands as at 

 Dolgelly. 



The Tremadoc Beds I have described in detail. Tremadoc in 

 Ynyscynhaiarn is the name-locality at which Sedgwick first dis- 

 tinguished Tremadoc Slates ; and, although the slates of the village 

 of Tremadoc with their pisolitic iron-ore are of Llandeilo age, this 

 need not affect the nomenclature. 



The Tynllan Beds or' Lower Tremadoc ' of Salter are thinly- 

 bedded rusty-grey slates with a harder cherty band near the top,, 

 and have characteristic white-weathering spots due to small 

 pyritous concretions. They are the Niobe horizon of Arenig, and 

 have not been recognized elsewhere. 



The Dictyonema Band, 15 feet thick, is a notable horizon. 

 I have discussed its importance and distribution in another place. 1 



The Moelygest Beds are a barren series of banded grey slates 

 or mudstones, containing enough pyrite to cover their weathered 

 surfaces with rust. They correspond exactly with the Bellerojyhon 

 Beds of Arenig. 



The Portmadoc Flags are the coarsest of the Tremadoc Slates. 

 They contain the debris of much felspathic material, and in the 

 west include considerable masses of ashy grit. They are the 

 Asaphellas Beds of Arenig. 



The Penmorfa Beds are micaceous and slightly calcareous. 

 They occur at Penmorfa as flaggy blue mudstones without much 

 cleavage, and elsewhere as thinly-bedded banded needle-slate. They 

 contain Shumardia and the Shineton fauna, 2 and with it occasional 



1 Geol. Mag. 1907, pp. 2G0 et seqq. 



2 C. Callaway, Quart. Journ. Geol. Soc. vol. xxxiii (1877) p. 652. 



