30 Notices of Memoirs—A. Coa—Ordovician, Cader Idris. 
NOTICHS OF MEMOTRS. 
I.—Tue Orpovictan Srquence In THE Caper Jpris Disrricr 
(MerionetH).1 By Arrnur Husperr Cox, M.Sc., Ph.D., F.G.S., 
and ALFRED Kinestey WELLs. 
EFERENCE was made to the work of previous observers, 
including Sedgwick, Ramsay, Cole and Jennings, 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— 
f 10. Talyllyn Mud- Grey blue-banded mudstones with 
Glenkiln-Hartfell . stones Amplexograptus arctus in the 
( lowest beds . - . 800 ft. 
9. Upper Acid or ‘Andesitic’ and rhyolitic ashes 
Craig-y-Llam and lavas . - 900-1,000 ft. 
Series 
8. Llyn Cau Mud- 
stones 500 ft. 
7. Upper Basic or Pillow lavas (spilites) with tuff and 
Pen-y-Gader chert bands ; . 3800 ft. 
Series 
Glenkiln with Upper } 6. Llyn-y-Gader Blue-grey mudstones with thin 
Lilanvirn Mudstones adinole -like bands and with 
and Ashes massive ashes above and below 
450 ft. 
5. DarkMudstones Frequently containing pisolitic 
ivon-ore  . ‘ oats 
4. Lower Basic or Pillowy spilitic lavas with inter- 
Llyn Gafr calated ashy and shaly bands, 
Series massive banded andagglomeratic 
ashes at the base . 1,500 ft. ° 
3. Didymograptus Dark slates with well-marked ash 
Lower Llanvirn | bifidus Beds bands in the lower portion 
500 ft. 
2. Lower Acid or Rhyolitic ashes with some slates 
MynyddGader above, rhyolitic lavas below 
Arenig . : : Series 300 ft. 
1. 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 albitized 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 Series, and it is noteworthy that the 
1 Read before the British Association, Section C, Manchester, 1915. 
