130 ME. H. LAPWORTH ON THE [Feb. I9OO, 



extent to prove that the unconformity between the Tarannon 

 Shales and the Lower Llandovery is so strongly marked as in these 

 southern areas ; but, as already suggested, it is probable that, 

 if the Rhayader Pale Shales were followed eastward, they would 

 be found in time to have completely overlapped the beds of the 

 Gwastaden Group. 



This epoch of subsidence and burial by overlap appears to have 

 been continuous in and around Wales up to Old Red Sandstone 

 times; for, in the same way, the Wenlock, Ludlow, and Old Eed 

 Sandstone beds all pass, in turn, over the outcrops of their under- 

 lying rocks on to those of greater antiquity. 



In this southern district, again, it is well known that a strong 

 unconformity exists between the Upper and Lower Llandovery 

 formations — an unconformity representing elevation, denudation, 

 and subsidence between the two periods of deposition. This is in 

 exact accordance with the results now r obtained in the Caban Coch 

 area. 



For a detailed zonal comparison of the rocks of the Rhayader 

 Series with those of the Lake District, Southern Scotland, and Sweden, 

 the reader is referred to Table II, facing p. 126, 



It is evident that in the Rhayader succession we have the entire 

 series of formations which answer to the Valentian Group of Southern 

 Scotland, a group which is universally admitted to include the 

 representatives of the Lower Llandovery, Upper Llandovery, and the 

 Tarannon Groups of Wales. In Southern Scotland this Valentian 

 succession contains extremely varied lithological types : a thin black- 

 shale series (the Lower and Upper Eirkhill) forming its two lower 

 divisions, and a massive grit or grauwacke series (the Gala Group) 

 its upper member. Rut the whole series is bound together, as an 

 unit, by the ascending sequence of the various graptolite-species that 

 mark its successive beds. The corresponding formations of Wales 

 and theW 7 est of England — the Lower Llandovery, Upper Llandovery, 

 and Tarannon Groups — have been studied hitherto in areas where 

 only one member existed in force ; or, if two or more members were 

 present, they varied in lithological characters, and were separated 

 by marked unconformity; while their fossils were almost wholly 

 brachiopoda, trilobita. and the like. In this Rhayader region, for 

 the first time in Southern Britain, do we find the Valentian succession, 

 in which the w^hole, or practically the whole, of the strata belonging 

 to the Lower Llandovery, Upper Llandovery, and the Tarannon 

 are in a grand sequence of rocks possessing a more or less common 

 lithological character and a fauna composed throughout of similar 

 palaeontological types. The whole Valentian succession of Rhayader 

 falls very naturally into three distinct members — the Gwastaden 

 Group or Lower Llandovery, the Caban Group or Upper Llandovery, 

 and the Rhayader Pale Shales or Tarannon; and each of these 

 members breaks up into subdivisions marked by characteristic grap- 

 tolites. It is, therefore, not unlikely that the Rhayader Series, in 

 which the sequence is so certain and the graptoiite-fauna so definite, 



