Geology of Mynydd Gader. 199 



are two principal anticlinal axes which follow in the main the 

 valleys of the Teifi and the Towy, and are named after these rivers ; 

 between them is an important syncline (the Central Wales Syncline) 

 which coincides nearly with the principal watershed of Central 

 Wales. 



Both the anticlines can be traced towards Pembrokeshire, where 

 they appear as important structures ; but they cannot be dis- 

 tinguished beyond the northern boundary of the area. The 

 syncline, on the other hand, becomes more important in a northerly 

 direction, but is lost towards the south-west. These structures 

 have a southerly pitch at the northern end of the district, and a pitch 

 in the opposite direction at the southern end. The variation in the 

 pitch accounts for the form of the outcrops. 



The peculiar correspondence between the evenly-curved courses 

 of these structures and the form of the coast-line of Cardigan Bay, 

 as also the relation of the structures to the other physical features, 

 are discussed. 



May 1st, 1912.— Dr. Aubrey Strahan, F.R.S., President, 



in the Chair. 



The following communications were read: — 



1. ' On the Geology of Mynydd Gader, Dolgelly ; with an Account 

 of the Petrology of the Area between Dolgelly and Cader Idris.' 

 By Philip Lake, M.A., F.G.S., and Prof. Sidney Hugh Reynolds, 

 M.A.,P.G.S. 



Mynydd Gader lies immediately south of the area described by 

 the authors in a previous paper (Q. J. G. 8. vol. lii. 1896, pp. 511- 

 21). The Tremadoc Beds are here succeeded by a group of rocks 

 which are, for the most part, of volcanic origin. These may be 

 divided into a Rhyolitic Series below and an Ashy Series above. 

 The Rhyolitic Series is formed chiefly of lava-flows ; the Ashy Series 

 consists mainly of volcanic ashes and slates, the ashes predominating 

 below and the slates above. Didymograptus bijldus occurs near the 

 base of the Ashy Series, D. murchisoni in the upper part. 



There is not sufficient evidence to show whether the Rhyolitic 

 Series is conformable or unconformable with the Tremadoc Beds ; 

 but, since it lies between the Dictyonema Zone and the Didymograptus- 

 hifidus Zone, it belongs presumably either to the Lower or to the 

 Middle Arenig. The Ashy Series extends from the Upper Arenig 

 to the Llandeilo. 



There are many intrusive masses of igneous rock, the largest 

 being that which forms the greater part of Mynydd Gader itself. 

 This intrusion is L-shaped in section — consisting of a horizontal 

 limb which cuts across the beds, and of a descending limb which 

 lies approximately in the bedding. 



The petrology of the igneous rocks, not only of Myn) dd Gader, 

 but also of the area described in the previous paper, is dealt with. 



