Revieivs — Geology of the South Wales Coalfield. 173 



( Orihograptus) truncatus (?) is assigned to either the upper part of the 

 zone of Dicranograptus Cling ani or the lower part of that of Pleuro- 

 graptus linearis in the Scottish Southern Uplands. As D. (0.) 

 truncatus var. abbreviatus is found in what appear to be corresponding 

 beds in the neighbourhood of Sedbergh, along with Dicellograptus 

 anceps, it may be suggested that the Shoalshook Limestone may 

 possibly be on the horizon of the last-named zone. 



The highest Ordovician strata, the Redhill-Slade Beds, exhibit 

 a northern and a southern type. Fossils are rare in the northern type. 

 The Silurian beds belong to the Valentian (Llandovery-Tarannon 

 Series). They seem in places to grade down into the highest 

 Ordovician strata, though elsewhere a conglomerate occurs at the 

 base. They are divided into three stages : these are, in ascending 

 order, the Haverford, Millin, and Rosemarket stages. The beds of the 

 last-named occur in a different tract to that showing the two former 

 stages, being developed on the south side of the coal-field. '•' It may 

 be observed that on the whole the Haverford stage corresponds to the 

 so-called Lower Llandovery, while the Rosemarket stage may be 

 matched with part of the Upper Llandovery, but there is difficulty in 

 correlating the Millin stage with either of these subdivisions, and it 

 probably forms a group which is only partly represented, if at all, in 

 the district of Llandovery." The rich fossil lists from the Valentian 

 beds of this district will be extremely valuable to future workers 

 among the Llandovery rocks. 



Passing to the Upper Palaeozoic rocks, we find the Old Red 

 Sandstone occurring both to the north and the south of the coal-field. 

 The rocks differ widely in their characters. The beds are divided 

 into a lower group consisting of red marls with sandstones; con- 

 glomerates, and breccias, and an upper group (the Skrinkle Sand- 

 stones). The Lower Old Red Sandstone is everywhere unconformable 

 to the older rocks, while the Upper Old Red Sandstone is followed 

 conformablj' by the Carboniferous strata. Both groups are absent 

 near Haverfordwest, owing to westerly overstep of the Carboniferous 

 rocks. The beds of the lower division have yielded Pteraspis and 

 some plants, including Psilophyton (?). 



The Carboniferous Limestone is also developed to the north and 

 south of the coal-field. A closer comparison can be made between 

 the northern beds and those further east, and also between the southern 

 beds and their easterly equivalents, than between the northern and 

 southern beds of the area under consideration. It is suggested that 

 the area of deposition of these rocks in South Wales shallowed north- 

 ward towards a coast with a general east and west trend. 



The series corresponds in the main with the Avonian Series of 

 Dr. Vaughan. The following zones are recognized in descending- 

 order : — 



Dibunophyllum zone (lower sub-zone D 1 alone known). 



Seminula zone. 



Syringothyris zone. 



Zaphrentis zone. 



Cleistopora zone. 

 Evidence is given in favour of a division into Lower and Upper 

 Avonian between the two subzones of the Syringothyris zone (where 



