136 H. HICKS 02J THE FOSSILIFEROUS 



out without any doubt the position of the beds in the series, and to 

 correlate them with those in other districts. 



As described in Mr. Marr's paper, the rocks have been faulted 

 down against very much older beds, and the stratigraphical evidence 

 of their position is wanting ; we have therefore to rely almost 

 entirely upon the fossils and some slight assistance which wo 

 are able to derive from lithological characters. The whole fauna, 

 speaking generally, has an Arenig look about it ; and even were no 

 species capable of being clearly made out, I should have no hesitation 

 in placing the beds in that group. It is, however, far more satisfac- 

 tory to be able to point out, from evidence derived from other districts 

 where the succession is perfectly clear, what the real position of the 

 beds must be as proved by the contained fossils. At St. Davids the 

 Graptolites D. bifidus and D. indentus are fouud only in the upper 

 Arenig rocks, and D. furciUatus and D. Murchisoni in the Lower 

 Llandeilo. According to Messrs. Hopkinson & Lapworth, this is 

 also the position of these fossils in the neighbourhood of Shelve in 

 Shropshire, and in Cumberland. 



Another of the fossils, Orthoceras caereesiense, is also found in the 

 Upper Arenig rocks at St. Davids ; and the genus Garyocaris is only 

 known to occur in the Skiddaw slates of Cumberland, associated 

 with the same Graptolites as arc found here. I think we are justi- 

 fied therefore in correlating these beds with the Upper Areuig 

 rocks elsewhere. 



In some respects the fauna would indicate a position intermediate 

 between the Upper Arenig and Lower Llandeilo, a passage from the 

 one into the other. However, on grounds mentioned in a former 

 paper to the Society, viz. that no beds so low are anywhere ox posed 

 in the neighbourhood of Llandeilo, and that these were distinctly 

 recognized in parts of North Wales by Prof. Sedgwick many years 

 ago, and called the Arenig group by him, I feel it is but right that 

 they should be included in that group. 



The lithological evidence also tends to bear out in a marked 

 degree the foregoing conclusions. Prom the fact that it was about 

 this time that volcanic action commenced in Wales and Cumberland, 

 the rocks in each of these areas assumed much the same cha- 

 racter ; specimens from St. Davids, Shelve, Cumberland, and near 

 Caernarvon are all bluish-black micaceous shales, with little or no 

 cleavage. In no other part perhaps in these early series have 

 we this character so universally shown ; and I attribute the cause 

 to the admixture of a considerable amount of fine volcanic material 

 with the ordinary sediment. Why these were not formed into 

 slates, as the previous and succeeding beds, may have been from the 

 presence of too little argillaceous in proportion to the amount of 

 volcanic material ; for they were subjected to the same amount of 

 pressure as those in which the cleavage is now most marked. These 

 beds are succeeded in the places mentioned by contemporaneous 

 tuff beds, the first of which we have evidence in the British area. 

 There can be no doubt in my opinion that a moderately deep sea 

 prevailed over these areas at the time that these beds were 



