S6 



SILURIAN. 



SIL URIAN. 



I. WALES AND WEST OF ENGLAND. 



The Silurian rocks of Wales, etc., are divided as follows : — 



Ledbury Shales. 

 Downton Sandstone. 

 Upper Ludlow Beds. 

 Aymestry Limestone. 

 Lower Ludlow Beds 

 Wenlock Limestone. 

 Wenlock Shale. 

 Woolhope Beds. 



AT H'll ( Tarannon Shales, 

 o-^- < Upper Llandovery. 

 Series. | t t i a 



\ Lower Llandovery. 



Ludlow 

 Series. 



Wenlock 

 Series. 



(Passage Beds.) 



Denbighshire 

 Flags and 

 Grits. 



Down- 

 tonian 

 Series. 1 



Salopian 

 Series.^ 



Valentian 

 Series.^ 



On the eastern borders of Wales, and in the adjoining English 

 counties, the Silurian rocks are developed near tjsk. May Hill, 

 Woolhope, Malvern, Abberley, and Tortworth ; ^ and from the 

 central part of South Wales, near Llandeilo and Llandovery, the 

 rocks stretch by Builth to Ludlow and Wenlock, and again by 

 Montgomery to the neighbourhood of Conway. They appear also 

 near Dudley. 



At one time the division between Upper Cambrian (Lower Silurian) 

 and Silurian proper (Upper Silurian) was taken between the Upper 

 and Lower Llandovery rocks. These rocks are now, however, 

 grouped together, as they are very intimately associated. Sir A. C. 

 Ramsay remarks that in mineral character the Lower Llandovery 

 rocks so strongly resemble the Upper Llandovery rocks, that up to 

 1856 no geologist had been able to distinguish between them. 

 Even then, when the section at Noeth Grug showed a lower group 

 beneath the Pentamerus beds, it was a task of great difficulty to 

 separate them.' The stratigraphical evidence, according to Prof. 

 Hughes, shows that Llandovery rocks rest unconformably on beds older 

 than any Llandovery rocks, but it does not seem to be shown by 

 reference to any localities that Upper Llandovery is unconformable 

 to Lower Llandovery. Nor is there any trustworthy list showing 

 what fossils are peculiar to Upper and what to Lower Llandovery ; 

 and while paleontologists cannot separate the fossils of the two 

 formations into well-marked groups, stratigraphical geologists con- 



^ These terms have been suggested by Prof. Lapworth : the name of Valentian 

 being derived from Valentia, a Roman division of the south of Scotland. 



- These districts were described in detail by John Phillips, Mem. Geol. Survey, 

 vol. ii. part I. 



' Geology of North Wales, edit. 2, p. 5. 



