424 



TRANSACTIONS OP SECTION C. 



2. Notes on the Granite Surfaces of Mount Sorrel. 



Bi/ Professor W. W. Watts, F.B.S. , 



It h IS been suggested that some of the grooved and polished surfaces known 

 in this circle and in certain other Midland localities may be due to wind action 

 in Pleistocene times. A recently discovered section at Moimt Sorrel showed 

 glacial striae trending N. 10° W. on a set of wind-grooves trending about E. 

 and W. Proof is thus given that the Triassic wind-grooves have survived actual 

 glaciation, and may thus be expected to have been able to resist other less drastic 

 methods of recent and Pleistocene denudation. 



3. The Ordovician Sequence in the Cader Idris District {Merionelh). By 

 Arthur Hubert Cox, M.Sc, Ph.D., F.G.S., and Alfred 

 KiNGSLEY Wells. 



Reference was made to the work of previous observers, including Sedgwick, 

 P^amsay, Cole and Jenning.?, Geikie. and Lake and Reynolds. 



The Cader Idris range is formed by a great escarpment of Ordovician igneous 

 rocks, facing northwards across Barmouth Estuary towards the Harlech dome. The 

 igneous rocks were for long regarded as being all of Arenig age. 



Re-examination of the area has shown the presence of four distinct volcanic 

 series among the Ordovician rocks, and the following descending sequence has been 

 established : — '*!(( 



((X.) Talyllyn Mud- Orey-blue banded mudstones with 

 \ stones Amplexograptnif nrctiia in the 



i lowest beds ... 800 ft. 



(IX.) Upper Acid or ' Andesitic ' and rhyolitic ashes and 

 Craig - y - Llani lavas . . ". 000-1.000 ft. 



Series 



(VIII.) Llyn Can 

 Mudstones 



Glenkiln-Hartfell 



Glenkiln with Upper 

 Llanvirn 



(VII.) Upper Basic or 

 Pen-y-Gader Series 



/(VI.) Llyn-y-Gader 

 ^, Mudstones and 

 ' Ashes 



500 ft. 



Pillow lavas (spilites) with tuff and 

 chert bands . . . .300 ft. 



Blue-grey mudstones with thin 

 adinole-like bands and with mas- 

 sive ashes above and below . 450 ft. 



(V.) Dark Mudstones Frequently containing pisolitic iron- 

 ore ' 80 ft. 



(IV.) Lower Basic or Pillowy spilitic lavas with inter- 



Llyn Gafr Series calated ashy and shaly bands, 



massive banded and agglomeratic 



\ ashes at the base . 1,500 ft. 



J (III.) Didymograp- Dark slates with well-marked ash 

 ( tus bifidus Beds bands in the lower portion .500 ft. 



/(II.) Lower Acid or Rhyolitic ashes with some slates 



Mvnydd Gader above, rhyolitic lavas below 

 -! Series 300 ft. 



(I.) Basement Beds . Striped arenaceous flags and grits 



100 ft. 



Unconformity 



Upper Cambrian . Tremadoc Beds 



Both acid and basic rocks occur as sills at numerous horizons. The basic rocks 

 are diabases of various types, all with the felspars considerably albitised and usually 

 with primary quartz. The acid rocks are soda-granophyres. The granophyre 

 intrusions cut and are later than the basic intrusions, and locally hybrid rocks appear 

 to have been formed along the junctions. No basic intrusions have been found 

 above the Upper Basic Volcanic Series, and no acid intrusions above the Upper Acid 



Lower Llanvirn 



Arenig 



