170 STRATIGKAPHICAL GEOLOGY 



slates. Southward they extend into Carmarthenshire, both Upper 

 and Lower divisions being well developed at Llandovery, but south 

 of that place they are cut out by faults. 



Northward the Rhyader facies is continued through Mont- 

 gomery, but the several members of the series become much thinner 

 till, near Bala and Corwen on the Dee, the whole is reduced to less 

 than 1000 feet. At Corwen it consists of the following beds : 



Tarannon shales .... 300 feet. 

 Graptolitic mudstones . . 300 ,, 

 Corwen grits .... 200 ,, 



The position of these beds is shown in Fig. 57. The Corwen 

 grits are grey and white sandstones with bands of coarse and 

 pebbly sandstone, and are believed to vary from 50 to 300 feet 

 in thickness. They contain few fossils, but Meristella crassa 

 and Pentamerus oblongus have been found, which are Lower 

 Llandovery species. Above them are soft bluish-grey or black 

 mudstones which contain Monograptus gregarius, M. tennis, and 

 Climacograptus normalis ; and these pass up into the pale Tarannon 

 slates. 



In North Denbigh the whole series is less than 500 feet thick, 

 but the zonal succession of beds has been more fully made out by 

 the Misses Elles and Wood, 6 these being : 



Beds. Zones. 



II Monograptus crenulatus. 

 Light-coloured shales | Monograptus crispus. 

 with black bands. \ Rastrites maximus. 

 I Monograptus Sedgwicki. 

 Grey shales, flags, and | Monograptus gregarius. 

 mudstones. \Mesograptus niodestus. 



Conway Castle grits and Bryn Dowsi flags, 150 feet. 



The lowest beds contain Diplograptus tamariscus, Climacograptus 

 normalis, and other species of Lower Llandovery graptolites. 



Salopian. When the Wenlock Beds are traced westward 

 from Presteign through Radnorshire both the limestones are found 

 to thin out, leaving a continuous series of mudstones and shales. 

 The Lower Ludlow shales also persist, but the Aymestry limestone 

 at their summit disappears. 



The subdivision of this mudstone series was practically impos- 

 sible so long as lithological differences only were relied upon as a 

 means of classification, but in 1880 Professor Lapworth drew 

 attention to the graptolite fauna of these beds, and expressed the 

 belief that " by the aid of the lowly graptolite the geologist of 

 the future will be able to read off the natural succession " of the 

 monotonous Silurian mudstones. This has been accomplished 



