Vol. 62.] TARANNON SERIES OE TARANNON. 649 



Llandovery contained the graptolitic zones of the Lower Birkhill 

 •of Scotland, his Upper Llandovery those of the Upper Birkhill 

 (with the exception of the zone of Rastrites maximus), while the 

 lihayadcr Pale Shales which followed contained fossils of the zones 

 of Rastrites maccimus and of Monogrctptus e.viyuus. 



Summarizing the facts and conclusions hitherto arrived at with 

 respect to the ' Tarannon ' in Wales, we may regard it as estab- 

 lished that the ' Tarannon -Shale ' formation, as shown on the 

 Geological-Survey map at Conway, includes the zones of Rastrites 

 ma.vimus and Monoyraptus e.viytms, and that the lowest graptolitic 

 zone of the succeeding Wenlock formation throughout Wales is that 

 of Cyrtograptus Murdhisoni. As yet, however, we have no proof that 

 the ' Tarannon' contains the third and highest graptolitic Gala zone, 

 namely, that of Cyrtograptus Grayaz; nor do we know whether the 

 ' Tarannon-Shale ' band, as mapped, includes strata of the same age 

 all along its course. Further, the discovery of Dr. Herbert Lap worth 

 in the ' Pale Shales ' of Rhayader of the two fossiliferous zones 

 which mark the ' Tarannon Shale' of Conway, far to the south and 

 west of the band mapped as Tarannon in Central Wales, carries 

 with it the implication that here the Tarannon extends much farther 

 westward than is shown on the published maps. 



About midway along the course of the ' Tarannon ' band, and 

 south-east of the village of Llanbrynmair, a long promontory formed 

 # of Denbighshire Grits and ' Tarannon Shale ' is mapped as running 

 out from the main mass of Denbighshire Grits and extending 

 southward from Llanbrynmair to Stay-a-little, a distance of some 

 6 miles. This is shown by the outcrops and dips to be a syncline, 

 its centre being occupied by Denbighshire Grits and its flanks 

 by the 'Tarannon,' while the surrounding and underlying strata 

 are mapped as continuous with the so-called ' Lower Llandovery,' 

 which spreads out over the main part of Central Wales. 



As in this district the ' Tarannon Shales ' fringe the Denbigh- 

 shire Grits on its eastern, western, and southern borders, there 

 is per unit of area a greater length of boundary-line available for 

 study than anywhere else along the course of the ' Tarannon ' band. 

 Further, this ought to be regarded as the typical area of the 

 ' Tarannon Shales,' for they received their name from the small 

 river of Tarannon, which rises near the centre of this basin-shaped 

 outlier and Hows south-eastward down its longer diameter. And, 

 finally, not only is this Tarannon promontory important as being 

 the area where we should presumably expect to find the typical 

 development of the 'Tarannon' strata; but here the sequence 

 appears from the map to be of more than ordinary simplicity, while 

 the physical features afford especially-favourable opportunities for 

 the study and examination of the rocks in the field. 



On these grounds Prof. Lapworth selected this district, many 

 years ago, as that in which the detailed lithological and palsBonto- 

 logical succession of the 'Tarannon ' formation should besought for; 

 and, after the puolication of Dr. Herbert Lapworth's paper on the 



2yl' 



