172 Reviews — Geology of the South Wales Coalfield. 



volcanic rock renders them easy of study ; moreover, they show 

 alternations of the ' shelly ' and ' graptolitic ' types of these strata. 



The Arenig beds are representative of the lower (Didymograptus 

 extensus) and upper (Z>. hirundd) zones so widely distributed elsewhere. 

 The beds have not been separately mapped on account of the thinness 

 of the D. hirundo zone, but are distinguishable on the grou©d. An 

 ash occurs in the extensus beds of the Henllan Amgoed district, and 

 a more important volcanic series consisting of lavas and tuffs 

 developed near Trefgarn Bridge is probably on the same general 

 horizon. The succeeding beds with Didymograptus bifidus are 

 separated from the Arenig Series and placed in what Dr. Hicks 

 termed the Llanvirn Series, characterized especially by 'tuning-fork 

 graptolites'. In the Haverfordwest district the Lower Llanvirn 

 beds (with D. bifidus) are alone represented, the Upper Llanvirn with 

 D. Murchisoni being absent. Nevertheless it is interesting to find 

 fossils hitherto referred to the Llandeilo, as Ogygia Buchi and 

 Calymene cambrensis, in these beds. Indeed, the detailed survey of 

 this area has shown the longer range of many other fossils of various 

 strata. 



The Llandeilo Series is divided into two groups of strata, the 

 Llandeilo Limestone and Elags below and the lower part of the 

 Dicranograptus beds above. These are dealt with in separate chapters. 

 The lower group rests on the, bifidus beds and decreases in thickness 

 from south to north, with a corresponding diminution of the fauna. 

 This suggests the existence of a land area to the south, with an east 

 and west trend. 



The Dicranograptus Shales are partly Llandeilo and partly Bala. 

 They are subdivided into the basal Hendre Shales, the Mydrim 

 Limestone, and the Mydrim Shales at the top. 



The Hendre Shales are thinner than in the district to the east, 

 partly owing to their lower part having passed laterally into the 

 Llandeilo Limestone and Flags. They have a definite Llandeilo- 

 fauna. The surveyors draw the line between Llandeilo and Bala at 

 the base of the Mydrim Limestone. This limestone has the 

 graptolite fauna of the Nemagraptus gracilis beds, which Lapworth, 

 when describing his classic sections at Moffat, correlated with part of 

 the Llandeilo Series. One is disposed, therefore, to doubt the 

 desirability of drawing the line where the Surveyors have placed it, 

 and to avoid confusion would prefer to have it placed at a higher- 

 horizon. 



The Mydrim Shales, as a result of Miss Elles' work in another 

 "Welsh area, have been divided into four graptolitic zones. 



The Bobeston Wathen Limestone with numerous corals shows 

 evidence of overstepping the underlying strata. It has elsewhere 

 been regarded as the upper division of the Caradocian division of the 

 Bala Beds, the succeeding strata having been referred to the Ash- 

 gillian division. With this the Surveyors appear to agree. 



The Shoalshook Limestone has long been known for the richness of 

 its fauna. It forms the base of the Ashgillian representatives, and 

 these Ashgillian strata are shown to rest unconformably upon the 

 lower beds. This limestone from the presence of Liplograptus 



