THE ANCIENT ROCKS OF ST. DAVID'S. 175 



In my paper on the Tremadoc rocks of St. David's, read Dec. 

 1872, I expressed the opinion, on pakeontological grounds, that the 

 Lower Arenig beds at St. David's were represented in North Wales 

 by the Upper Tremadoc rocks. The examination of several sections 

 in Carnarvonshire during last summer, under the guidance of Prof. 

 Bamsay and Mr. Homfray, further confirmed my opinion ; and I now 

 feel convinced that the Upper Tremadoc beds should be classed with 

 the Arenig group. 



The Lower and Upper Tremadoc rocks are quite distinct, and very 

 unlike palaeontologically — -the fauna of the former, like that of the 

 Tremadoc group at St. David's, being closely allied to that in the Lin- 

 gala-nags, and that in the latter to the Arenig group. In the Arenig 

 district proper the lower black beds of Prof. Sedgwick's Arenig 

 group are undoubtedly, in regard to their position and their fossils, 

 identical with the Upper Tremadoc as exhibited further west ; but 

 there is a slight lithological difference observable, and this probably 

 has prevented their being correlated hitherto. 



The sections at Portmadoc, in the Ffestiniog mountains, and to 

 the west of the Arenig mountains, show in each case the following 

 order of the strata : — 1. Black shales (Lingula-flags), with Olenus &c. 

 2. Iron-stained slates and flags (Lower Tremadoc), with Niobe, Psilo- 

 cephalus, Neseuretus, Conocoryphe, &c. 3. Dark slates (Garth beds), 

 with AsapJius Homfrayi, Ogygia scutatrlv, Angelina, Conixlaria, &c, 

 and, in the Arenig district, some dendroid Graptolites. 4. A thick 

 grit-bed with no fossils. 5. Dark fine-grained slates (Ty-Obry 

 beds), with JEglina,] Asaphus, Calymene, Trinucleus, Dionide, 

 Denclrograptus, &c. 6. Ash and tuff beds, inters tratified with blue- 

 black fine slate. It is easy at once to recognize, in the order 

 given here, a likeness to the series at St. David's ; indeed, the only 

 difference observed is the occurrence of a thick bed of grit in North 

 Wales, at the part where the Middle Arenig comes in at St. David's. 

 This grit-bed is preceded and succeeded by fine deep-sea deposits ; 

 and the change in the character of the sediment is particularly rapid, 

 showing that the sandstone was probably deposited in a deep sea 

 by tidal action, and not a shore-accumulation. There is, as already 

 mentioned, a slight difference also in the character of the sediment in 

 the Middle Arenig at St. David's from that in the Lower and Upper 

 series, and narrow bands of sandstone are intercalated with the 

 slates ; but the St.-David's area seems on the whole to have been 

 almost out of the influence of the physical cause, whether tidal or 

 otherwise, which produced this sudden change in the North-Wales 

 series ; and hence we have deposits at this period at St. David's with 

 a fauna unknown there. The other St.-David's faunas resemble, in 

 many particulars, those found in the series in North Wales. In 

 Shropshire the order of the beds is almost similar to that observed in 

 North Wales, and the Stiper Stones are doubtless, as first suggested 

 by Mr. Salter, the equivalents of the grit beds in Carnarvonshire. 

 Hitherto the black beds under the Stiper Stones have proved almost 

 bairen of organic remains, and they cannot be correlated with other 

 series except by position ; but in the beds immediately upon the 



