130 ME. GAEDINEE AND PEOE. EEYNOLDS ON THE [May I909, 



The following are the possibilities with regard to the red 

 felsite : — 



1. That it may be intrusive either {a) as a boss like the green felsite, forcing 

 its way indifferently through all the pre-Bala sedimentary rocks, or (b) as a 

 laccolite or sill between the Arenig rocks on the east and the Llandeilo rocks 

 on the west. The extremely sinuous outcrop tells against both these views, as 

 does the complete absence of any sign of metamorphism in the limestones 

 which are so frequently exposed near the felsite. The 'intrusive' view also 

 affords no explanation either of the frequent juxtaposition of the limestone 

 to the felsite, or of the great abundance of felsite-fraginents in the tuffs and 

 limestone-breccias. 



2. That it may form one or more flows of Lower Llandeilo age — may, in fact, 

 be the earliest event of Llandeilo time of which the district affords a record. 

 According to this view, which, although not free from difficulties, seems to us 

 to be the most satisfactory one available, the sinuosity of the eastern margin 

 would simply depend on the extent to which the felsite had been removed by 

 denudation from the Arenig rocks, and that of the western margin on the extent 

 to which the Llandeilo tuffs had been denuded from the surface of the felsite. 

 If almost immediately after the outpouring of the felsite the limestones were 

 deposited, and then by explosive outbursts were disrupted, giving rise to the 

 limestone-breccias, their frequent occurrence close to the felsite and their non- 

 metamorphosed condition would be explicable. The abundance of felsite-frag- 

 inents in the tuffs and limestone-breccias also finds a ready explanation on this 

 hypothesis. The chief difficulty in the way of its acceptance is the great amount 

 of variation in width of outcrop, and hence presumably in thickness, of the 

 felsite, though the difficulty may partly be due to the repetition of the rocks 

 by faulting or folding. Perhaps also the non-vesicular character of the red 

 felsite and the rare occurrence of flow-structure may be taken as objections to 

 this view. 



Our conclusions with regard to the felsitic rocks may be sum- 

 marized as follows : — 



1. That the great red felsite (rhyolite)-mass forms a series of lava-flows of 

 Lower Llandeilo age, which are probably penetrated here and there by minor 

 felsite-intrusions. 



2. That the remainder of the felsites are intrusions, some certainly, others 

 probably, of post-Bala and pre-Carboniferous date, all belonging to the same 

 general period, and that these intrusions take the form of: — 



(i) bosses, as, for instance, the green and brown felsite-masses ; 

 (ii) dykes, as, for instance, the long intrusions which enter the south- 

 western corner of the map, and the small patches scattered along its 

 western border. 



(6) The Intrusive Andesitic Rocks. 



At several points in different parts of the district small intrusions 

 of fine-grained non-porphyritic rocks of intermediate character are 

 met with. Without microscopical examination these rocks are not 

 easily distinguished from certain of the felsites and fine ashes, and 

 it is not unlikely that other small masses may occur. 



The following are the exposures : — 



(1) A mass, exposed at various spots along a distance of about half 

 a mile, runs in a north-n or th- easterly direction from a point 

 nearly half a mile north-north-east of Tourmakeady Lodge as 

 far as Stream C. It is a dark-green, generally compact rock, 



