Vol. 68.] THE GEOLOGY OF MYNTDD-T-GADEK. 347 



(2) The stratified Rocks. 



The stratified rocks of the area may be divided as follows, in 

 descending order : — 



(5) Ashy Series, consisting chiefly of interbedded slates and volcanic ash, 



with occasional andesitie and rhyolitic lavas. 

 (4) Rhyolitic Series, formed almost entirely of rhyolitic lavas and rhyolitic 



ashes. 

 (3) Tremadoc Slates. 



(2) Upper Lingula Flags or Dolgelly Beds. 

 (1) Middle Lingula Flags or Ffestiniog Beds. 



(1-3) The Lingula Plags and Tremadoc Beds are exposed 

 only in the northern part of the area, and coucerning them we have 

 nothing to add to our former account. 



(4) The Rhyolitic Series follows the Tremadoc Beds, but the 

 relations of the two are not clear. The Dictyonema Zone is seen near 

 Gilfachwydd, east of Llyn Gwernan. In the little gully south of 

 the farm slaty beds continue for some distance ; but exposures are 

 poor, and we found no fossils. Towards the top of the gully the 

 Rhyolitic Series appears. There is no evidence of unconformity ; 

 but it would be almost impossible to detect an unconformity here, 

 unless it were very strongly marked. 



The Rhyolitic Series covers a considerable area on the north- 

 west of the Mynydd-y-Gader dolerite ; but its base, following the 

 general direction of the beds below, strikes against the edge of the 

 dolerite south of Tan-y-Gader, and the series accordingly disappears. 

 Narrow strips, however, appear at intervals beneath the dolerite all 

 along its northern boundary. In these strips the dip, wherever it 

 can be determined, is almost due south, and the dolerite rests 

 unconformably upon the edges of the beds. 



On the southern side of the dolerite the Rhyolitic Series is 

 absent in the extreme west ; but everywhere else it is the Rhyolitic 

 Series that lies next to the dolerite. The dip is almost invariably 

 nearly due south, except at the eastern end of the intrusive mass, 

 where it becomes south-easterly. The angle of dip is usually high. 



The Rhyolitic Series consists partly of lavas and partly of ashes, 

 but both lavas and ashes are always acid. The petrology is 

 described in § III of this paper : it may be noted here, however, 

 that on the whole the lower part of the series appears to be ashy, 

 while in the upper part the lavas, sometimes banded and sometimes 

 compact, predominate. 



(5) The Ashy Series. — The lavas forming the upper part of 

 the Rhyolitic Series are everywhere followed conformably by a group 

 of ashes and slates with occasional beds of lava. In the eastern 

 part of the map the distinction between the two series is clearly 

 defined, and the boundary is easy to trace. The ashes usually 

 weather much darker than the rhyolites, and their fragmental 



