Vol. 56.] THE SILURIAN SEQUENCE OF RHAYADER. 67 



7. The Silurian Sequence of Rhayader. By Herbert Lapworth, 

 Esq., Stud.Inst.CE., E.G.S. (Read November 22nd, 1899.) 



[Plates VI & VII —Vertical Sections & Geological Map.] 



Contents. 



Page 

 I. Introduction 68 



(1) The Ordovician and Silurian Complex of Central Wales. 



(2) The Rhayader District. (Topographical Map on p. 70.) 



(a) General Characteristics. 



(b) Historical Summary. 



II. Stratigraphical Relations of the Rocks of the Rhayader District ... 74 

 (A) The Gwastaden Group 74 



(a) Description of the Sections in the Rhayader Valley. 



(1) Typical Section through Gigrin Prysg and the Wye. 



(a) Cerig Gwynion Grits. 

 (&) Dyffryn Flags. 



(c) Ddol Shales. 



(d) Gigrin Mudstones. 



(2) Confirmatory Sections. 



(a) Dyffryn Wood and Woodlands. 



(b) Esgair-rhiw. 



(b) Supplementary Sections through Castle Hill and the 



Eastern Portions of the District. 



(1) Castle Hill. 



(2) Vaynor. 



(c) Exposures in the Western Areas. 



(1) Allt-y-bont. 



(2) Corn Gafallt. 



(3) Allt Goch. 



(4) Nant-y-Gro. 



(d) Additional Notes on Supplementary Exposures. 



(1) Cerig Gwynion Grits. 



(2) Dyffryn Flags. 



(3) Ddol Shales. 



(4) Gigrin Mudstones. 



(e) Summary. 



(B)_The Caban Group 99 



(a) Typical Section at Caban Coch. 



(1) Caban Conglomerates. 



(2) Gafallt Beds. 



(3) Exposures in Caban Coch Gorge. 



(b) Confirmatory Sections through Corn Gafallt Hill. 



(1) Clap-round. 



(2) Corn Gafallt. 



(3) Pen-y-rhiw. 



(4) Dol-Ifor. 



(c) Additional Notes on the Caban Synclinal. 



(d) Discussion and Summary. 



(C) The Rhayader Pale Shales 119 



III. General Summary 124 



(a) The Rhayader Secpience. 



(b) Comparison with the Deposits of other Areas. 



IV. Appendix. On New Species of Graptolites from the Rhayader District. 131 



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