94 SILURIAN. 



rather pass by lithological gradations into strata of a shaly character. 

 In some areas fossils are absent or very scarce in the grits, a few 

 fragments of Encrinites or of bivalve MoUusca alone showing 

 that they are fossiliferous. In other places, however, fossils are 

 plentiful, as near Conway, at Plas Madoc, near Pentre Voelas, 

 Craig-hir, etc. 



Amongst the fossils may be mentioned Phacops Doivningice, 

 P. caudalus, Calymene Bliimenbachti, Rhyncho7ieUa nucula, Stropho- 

 viena, LeptcE.na, Cardiola interrupta, Euomphalus, Murchisonia, Belle- 

 rophon expansiis, Orthoceras prmaviim, etc. Mojiograptus {Grapto- 

 liihus) priodon also occurs ; and Land-plants, the oldest yet known, 

 have been found by Dr. H. Hicks in grits exposed in a slate- 

 quarry at Pen-y-glog, near Corwen.^ These are the Penyglog Grits 

 described by Prof. Hughes.^ 



The following local divisions have been made : — 



^ , . , , . „ ., f Dinas Bran Beds and Llansannan Shales. 

 2. Denbighshire Grits. 5 p, j ^nts. 

 I. Denbighshire Flags. |pen;|io| Flags. 



TheDenbighshire grits and flags succeed theTarannon shales, and, 

 interstratified with slaty shales, they form the base of the Wenlock 

 Series. They run from north to south, in a long sinuous and some- 

 times broad strip, from the mouth of the Conway to Melenydd. 

 East of Bala Lake they lie in a trough, from two to four miles wide, 

 and the Tarannon shales and older rocks of the Berwyn hills 

 rise from underneath their eastern boundary. North of the Berwyn 

 hills, between Llangollen and Corwen, the Denbighshire grits, more 

 shaly in character, overlie the Tarannon shales ; and in the valley 

 of the Vyrnwy, and eastward by Welshpool and the Long-mountain, 

 and round the older rocks of the Shelve and Corndon country, the 

 sandy character of the base of the formation disappears. It may, 

 indeed, be stated that in the Long-mountain, north of Montgomery, 

 the Wenlock Series exhibits a transition from the calcareous 

 development in Shropshire to the arenaceous type of Denbigh- 

 shire.^ In Radnorshire, ten or twelve miles north of Builth, the 

 Denbighshire grits die out, but their equivalents in a more shaly 

 form are believed by Mr. Aveline to strike into South Wales.'* 

 Many fossils have been found in the Llansannan Shales. 



In the Clwyd Valley, above the Pale Slates and Graptolitic 

 Mudstones, Prof. Hughes notes the following beds : — 



Grey gritty, and concretionary Sandstones. Denbighshire Grits. 



Sandy Mudstones and grey banded Sandstone. \ Denbighshire Flags. 

 Flaggy Shales and Sandstone. ) *= 



In the flaggy beds at Pontyrddol, near St. Asaph, and on Moel 

 Fodia, the following fossils were obtained : — Fatwsites fibrosus, 



1 Q. J- xxxvii. 482 ; xxxviii. 97. 



- Q. J xxxiii. 207. 



:' W. W. Watts, Q. J. xli. 537. 



* Ramsay, Geol. N. Wales, edit. 2, p. 14. 



