348 MR. p. LAKE AND PEOF. S. H. EEYNOLDS ON [Sept. I912, 



character is very clearly shown. Even here, however, there are 

 one or two hands of nodular rhyolite in the midst of the ashes. 



Towards the west the difference between the two series is much 

 less marked, and it becomes a very difficult matter to separate 

 them. The colour of the ashes is no longer distinctive, the frag- 

 mental character is much less conspicuous, and there appear to bo 

 more intercalated lavas. So great, in fact, is the difficulty of dis- 

 tinguishing between the two series at the western end of Mynydd-y- 

 Gader that the boundary drawn in this part of our map (PI. XXXV) 

 must be looked upon as tentative only. 



In the eastern part of the area the series begins with a rather 

 dark tuff, often with large fragments included. One or two lenti- 

 cular bands of rhyolite occur within the tuff, and upon it lies a bed 

 of black slate, which in a trial-working near the Afon Aran contains 

 Didymograptus hifichis Hall.^ 



Above this slate the series consists of alternating beds of tuff and 

 slate, with andesitic lavas appearing towards the south of the area ; 

 but, owing in part to the presence of intrusive sills, it is very 

 difficult to map individual beds for any considerable distance. 



At the western end of Mynydd-y-Gader is an oblique fault with 

 its downthrow on the west, and on the downthrow side the rocks, 

 consist chiefly of slate with occasional beds of tuff and of rhyolitic 

 and andesitic lava. The slates have been extensively quarried at 

 Penrhyn-gwyn, where we found Orthograptus calcaratus var.pWscies 

 Elles & Wood ^ and Didymograptus murchisoni (Beck).^ 



Age of the Ehyolitic and Ashy Series. — Since the base 

 of the Ehyolitic Series lies some distance above the Dictyonema 

 band and its top is some distance below the zone of Didymograptus 

 hifidus, its age is fairly accurately determined. It belongs pre- 

 sumably either to the Lower or to the Middle Arenig. 



The Ashy Series, as shown upon the map, includes both the 

 Didymograptus-hifidus Zone and that of Didymograptus murchi- 

 soni. It belongs, therefore, to the Upper Arenig and the Lower 

 Llandeilo. But it is in the lower part that the ashes predominate, 

 while the upper part, the zone of D. murchisoni, consists chiefly of 

 slate. 



(3) The Forms and Relations of the Intrusive Masses. 



In our previous paper we described the form and relations of the 

 masses of dolerite in the immediate neighbourhood of Dolgelly. 

 The only observation that we need add to the account which we- 

 then gave is that in the midst of the dolerite south of Bryn-y- 

 gwyn there is a considerable area of volcanic rocks. 



Of the remaining masses the largest and most important are 

 those of Pare and of Mynydd-y-Gader, and it is to these that we 

 have devoted most attention. 



1 We are indebted to Miss G. L. Elles, D.Sc, for the identification of thes& 

 graptohtes. 



