264 PROF. A. H. COX AST!) MR. A. K. WELLS OX THE [vol. lxxvi, 



thickness of beds. Unfortunately, the exact position of this second 

 Dictyonema Band with regard to the Asaphellus Beds of Llvn 

 Wylfa cannot be determined, as the strata that presumably in- 

 clude the Dictyonema Band are hidden beneath the lake and 

 the bordering alluvium. There can be no doubt, however, that 

 this second Dictyonema Band occurs stratigraphically above the 

 Asaphellus Beds, and is therefore quite distinct from the first 

 Dictyonema Band which lies well below the Asaph ell us Beds. 



(vi) The Upper Pencil-Slates. — The highest Tremadoc Beds 

 in this district comprise a group of softer strata, with an irregular 

 cleavage which cavises the rocks to crumble into small fragments, 

 or sometimes to split into long splinters or pencils. Fossils are 

 hence difficult to obtain, and so far only a few specimens have 

 been collected. BellerqpTion sp. was obtained at a point on the 

 south-east side of Llyn Wylfa close underneath the basal grit of 

 the Ordovician, and a solitary example of Asaphellus (?) from a 

 locality a few yards north of the "Waen-Fechan sill. Owing to the 

 lack of fossils, it remains uncertain for the present as to whether 

 these beds represent the Angelina Beds of other areas. 



The total thickness of the Tremadoc Slates, as developed in the 

 Dolgelley district, amounts to a little over 900 feet. The corre- 

 lation of the beds is discussed in a later section (§ V, p. 315). 



Relation of the Ordovician to the Cambrian Strata. 



Throughout the western part of the district mapped, the Tre- 

 madoc Slates are overlain by a grit which forms the local base of 

 the Ordovician strata. The change in the type of deposit takes 

 place quite abruptly. The sudden change may indicate a break 

 between the two series, but the evidence so far obtained does not 

 suffice to prove the presence of an actual unconformity. In the 

 first place, the exact dip of the higher beds of the Tremadoc Slates 

 cannot, as a rule, be determined owing to cleavage. Therefore, 

 even in the case of good exposures, it becomes impossible to 

 ascertain whether or no there is actual discordance between the 

 dip of the slates and that of the overlying grits. It is, however, 

 certain that if there is any discordance it can only be very slight. 

 Secondly, although the junction has been mapped over a distance 

 of several miles, the mapping has failed to demonstrate the presence 

 of an unconformity. It is true that over more than half the 

 distance evidence on this point is lacking, owing to the prevalence 

 of faults and intrusions. Allowing for such disturbances, however, 

 the exposures are clear over a distance sufficient to show that there 

 can be no great discordance of dip and strike between the two 

 series. The relationship between the Cambrian and the Ordovician 

 Systems in the Arthog district is, therefore, comparable with that 

 which obtains in the Arenig district, where the unconformity is so 

 slight that its presence was only proved by detailed mapping over 



