part 3] LOWER PALiEOZOIC of akthog— dolgelley. 319 



A minor point is the occurrence of Dictyonema sociale at two 

 distinct horizons in the Tremacloc Slates. The Lower Dictyonema- 

 Band immediately overlies the Niobe Beds, and is therefore on the 

 same horizon as the band at other well-known localities ; the Upper 

 Band occurs about 500 feet higher in the series, and above the 

 Asaphelli/s Beds. 



The relationship of the Ordovician System to the underlying 

 Tremadoc is obscure. Although neither actual transgression nor 

 discordance of dip can be observed, the lithological characters of 

 the beds are quite distinct, and they suggest the presence of a 

 break in the sequence. 



The Ordovician succession commences with a thin Basement 

 Series, consisting of felspathic grits and quartzites, with associated 

 banded grits and flags which show alternations of argillaceous and 

 arenaceous material. 



These Basement Beds are succeeded by a volcanic series, the 

 Lower Acid Series, which shows considerable lateral variation. 

 The series is thickest in the east, where it consists mainly of 

 rhyolitic tuffs, with rhyolitic lavas in the south-east. The tuffs 

 and lavas thin away rapidly westwards, and coincident with this 

 thinning, slate-bands — the Pont Kings Slates — appear among the 

 volcanic rocks in the western part of the district. 



The slates yield a mixture of extensiform and tuning-fork 

 graptolites, showing them to be in part of Arenig age (zone of 

 Didymograptus hirundo) and in part of Lower Llanvirn age. 



The JBifidus Beds consist mainly of fossiliferous slates separated 

 into two bands — the Moelyn and the Orogenen Slates — by a group 

 of ashy grits, the Bryn Brith Beds. In the eastern part of the 

 district these grits overlap the Moelyn Slates, and rest directly 

 upon the rhyolitic rocks of the Lower Acid Series. The peculiar 

 lithological characters of the group and its relations to underlying 

 groups are probably accounted for by deposition against the flanks 

 of a volcanic cone which was undergoing rapid erosion. 



A great thickness of volcanic rocks succeeds the main mass of' 

 the JBifidus Slates. The volcanic rocks begin with massive ashes 

 — the Cefn Hir Ashes — of andesitic composition. Thin shale-bands, 

 still yielding Didymograptus bifidus are associated with these 

 ashes. The Cefn Hir Ashes are followed by the Lower Basic Series, 

 a great thickness of spilites (often as pillow-lavas), with associated 

 ashes and subordinate slate-bands. 



The sequence of these various Ordovician deposits is essentially 

 similar to that at Arenig Mountain, the main difference between 

 the two areas being the greater thickness at Dolgelhy of the 

 Lower Acid Series and of the JBifidus Beds. A close comparison 

 may also be drawn between the Llanvirnian rocks of the Arthog- 

 Dolgelley district and those of Shropshire and South Wales. 



Intrusive rocks, both acid and basic, occur at all horizons in 

 the stratigraphical succession, in the form of sills or laccolites. 

 Individual sills tend to trangress northwards on to lower strati- 

 graphical horizons. 



