Reports and Proceedings — Geological Society of London. 4-3 

 The author establishes the following sequence : — 



Ehayader Group (Pale Shales) . 



( 



Caban 

 Group. 



Gwastaden 

 Group. 



Gafallt Beds 



i Cabau 



Conglomerates. 



Gigriu 



Mudstoues. 



Ddol Shales. 



DyfPryn Flags. 



Cerig Gwyuiou 

 Grits. 



Blue-black Shales. 



( Gafallt Shales. 



\ 3£. Sed(/wicki-grits. 



i Upper Conglomerate. 



' Intermediate Shales. 



( Lower Conglomerate. 



Pale Grey Mudstones. 

 Zone of M. cuiirolicfiis. 

 Calcareous Nodule-beds. 

 Zone of M.Jimbriatus. 



,, M. Cl/pllHS. 



,, Jll. tenuis. 

 I). iiwdestKS-^iigs. 

 Rottenstone Beds. 

 Micaceous Flas's & Grits. 



Gala and Tarannon 

 Group. 



Upper Llandovery, 

 May Hill, and " 

 Upper BirkhiU. 



Lower Llandovery 



and 

 Lower Birkliill. 



Bala and Hartfell. 



No single section, taken in any direction across the Ehayader 

 country, shows the complete succession. In any area some one 

 gi'oup is always better developed than others ; and some portion of at 

 least one group is certain to be missing. The lowest, or Gwastaden 

 Group, has a maximum thickness of over 1,800 feet. It is underlain, 

 apparently conformably, by highly cleaved dark-blue shales, and 

 overlain unconformably by both the Caban and Rhayader Groups. 

 The base of the Gwastaden Group is formed of a thick mass of 

 grauwackes, which thin to the east and thicken to the west. They 

 contain ClimacogrnpUis, and pass up into mixed flags and grits with 

 Clirnacograptus and Diplograptus. These are succeeded by shales 

 and mudstones in which the first Monograptidse appear. These 

 become dominant in the upper part. 



The succeeding Caban Group has a maximum thickness of 

 1,500 feet. Its lower division consists of two massive con- 

 glomerates, separated by shales ; its higher division is made up 

 of fine-grained grits, shales, and flags. Each member of the group 

 is overlapped to the east by the next subgroup above it, until 

 eventually the whole group disappears beneath the Ehayader Pale 

 Shales, which in the eastern areas rest directly on the Gwastaden 

 rocks. 



The Ehayader Group consists of pale-green, blue, and grey shales 

 and mudstones, which overstep on to the Gwastaden beds ; and may 

 possibly pass completely over them, and rest on the dark shales 

 farther east. 



After the Gwastaden rocks were laid down, the sea-floor appenrs 

 to have been elevated and denuded, a hollow being scoured out to 

 the eastward. Eapid sinking followed, and the sea filled the hollow 

 with the Cabati sediments, practically levelling it up by the time 

 that the deposition of the Pale Slates began. 



