part 2] PALAEOZOIC eocks of the llais^gollen district. 205 



the Nant-Arcldau Valley (Nant-y-Pandy), south of the Dee-side 

 Slab-quarries. Unfortunately, our collecting has not provided 

 material enough to confirm or disprove the existence of this zone. 



On the northern flank of the synclinorium, there appears to be a 

 complete upward sequence from the Wenlock Series into slates 

 with thin silty sandstones, but there is no reliable palaeontological 

 evidence b}^ which to distinguish the two formations. There is 

 here, above the doubtful series, a well-marked slate-band (the 

 Pentre-Dwfr Slates) the outcrop of which we have tried to 

 represent on the map (PI. Y). The slates are not recognizable 

 south of the Dee : they may, however, be cut out by the faulting 

 mentioned above. As roofing-slates they have good weathering 

 properties, but are too thick for modern requirements. The 

 cleavage passes indiscriminately through mudstone and laminated 

 silts in which uncleaved calcareous concretions are frequent. The 

 slates pass upwards into an indefinite series of slates with thin 

 silty sandstone-bands. 



The succeeding * Slab Horizon ' is traceable throughout the 

 synclinorium with fairly constant lithological characters. The 

 subdivision is essentially composed of rapidly alternating bands of 

 cleaved mudstone and uncleaved laminated silt. When not broken 

 up by close jointing, the rock splits readily between the cleaved and 

 uncleaved layers into huge slabs of a uniform thickness, and has 

 formerly been extensively quarried for slate-slabs, stone-cisterns, 

 flags, etc.^ As the graptolite fauna of this horizon has been shown 

 by Dame Ethel Shakespear ^ to be that of the JKonogrcqDtus- 

 nilssoui Zone, and, as other striking features of the fauna (the 

 local abundance of ' Actinocrinus'' pitlcher^ and of ' Orthoceras 

 primcevum'') have long been known, it is unnecessary to go into 

 further details. Our fossil-collecting confirms the reference of the 

 slabs to the nilssoni zone. 



The uppermost subdivision of the Gl^ai-Dyfrdwy Group is 

 composed of fairly thin-bedded shales and silty mudstones or sand- 

 stones, occasionally^ micaceous, more or less perfectly cleaved. 

 The sandstones are sometimes internally crumpled, and the shales 

 frequently show concentric staining very similar to that seen in the 

 Moughton Whetstone of the Austwick Valley (Yorkshire). The 

 group as a whole is rather conspicuously unfossiliferous, but a few 

 graptolites have been found on the hills south-west and south of 

 Glyn-Dyfrdwy and elsewhere, which suggest that the rocks form 

 part of the Monograptiis-nilssoni Zone. 



There are fine exposures in this group on the hillside north 

 of the bridge over the Dee at Glyn-Dyfrdwy, and north-west of 

 Dolywern near Glyn-Ceiriog. 



In the latter area the subdivisions of the Glyn-Dyfrdwy Group 



^ Flags of this rock, with the characteristic fossils, have been found in the 

 Roman Camp near Capel-Curig-. 

 " Q. J. G. S. vol. Ivi (1900) p. 446. 

 '^ D. W. Roberts, Trans. Edin. Geol. Soc. vol. i (1870) p. 329. 



