488 



H. HICKS ON THE REMAINS OE PLANTS FROM THE 



group. The accompanying section (p. 487) will explain more fully 

 the succession in this area and the actual position of the chief heds 

 containing the plant-remains. 



The evidence of the geological horizon, derived from the fossils, is 

 at present imperfect ; for the animal remains found in the slates, in 

 association with the plant-remains, are chiefly Graptolites, though 

 fragments of Encrinites, species of OrtJioceras, and some Brachiopods 

 are occasionally found. The following species of Graptolites were 

 recognized by Mr. Hopkinson, in a collection made by him and my- 

 self at different horizons in this quarry in 1875, viz. Cyrto- 

 graptus Murchisoni, Monograptusjiriodon, M. Sedgwickii, M. spiralis, 

 M. vomerinus, M. Halli, and Retiolites Geinitzianus*. 



These forms, he considered, were " characteristic of beds at the 

 summit of the Coniston Mudstones, or base of the Coniston Flags 

 in the Lake district. The abundance of Graptolites found in these 

 beds would tend to show that the deposits, for the most part, were • 

 thrown down in a tolerably quiet sea. 



It seems, therefore, so far as the evidence can be read at present, 

 that this immediate area was not greatly affected by the physical 

 changes which occurred in the neighbouring areas at the close of 

 the Bala epoch — that if uplifted above sea-level, it must have been 

 previous to the deposition of the Corwen Grits, as shown by Prof. 

 Hughes. The physical break, therefore, if it exists here at all, must 

 be placed at that point, and not, as formerly supposed, at the base of 

 the so-called Tarannon Shales. There is, however, no visible uncon- 

 formity between the Lower and Upper Silurians anywhere in the 

 sections in this neighbourhood; and it is quite possible that the area 

 may have remained under water during the whole of the Mid- 

 Silurian epoch f. The parts raised above sea-level were chiefly to 

 the south-east, south, and north-west of this area. I am inclined to 

 think that there was not a very extensive land area, but numerous 

 islands, some of them of volcanic origin. They reached undoubtedly 

 as far as Shropshire to the S.E., and to Caernarvonshire (and probably 

 beyond it) to the N.W. There is no satisfactory evidence to show 

 that they extended much further to the S.W. than the neighbour- 

 hood of Builth, as the deposits apparently accumulated uninter- 

 ruptedly during this time in part at least of Caermarthenshire, in 

 Pembrokeshire, and in Cardiganshire. This is the only way in which 

 we can account for the presence in those areas of some thousands of 

 feet of beds between the topmost Bala and the "Wenlock series, and 



* Mr. C. Lapworth has also kindly examined a small collection made by me 

 recently in the same quarry, and mentions the following forms as recognizable. 

 They were chiefly collected from the middle bands of slate and above the beds 

 with nodules and anthracite: — Retiolites Geinitzianus, Cyrtograptus Murchisoni, 

 Monograptus vomerinus, M. personatus, M. priodon (and vars. riccantonensis, 

 Lapw., and Flemingii, Salt.). He states, as to the correlation of these beds with 

 those in other areas, that, so far as the evidence derived from the above Grap- 

 tolites can be made out, they would occupy a position equivalent to the "lower 

 zones of the Wenlock shale of Shropshire and the west of England." 

 t See paper by author, Brit. Assoc. Eeport, 1875. 



