part 3] lower palaeozoic or arthog-dolgelley. 305. 



Llyn portion of the Cader-Idris range, cuts nearly, but not quite, 

 into this same Upper Acid Series. Now, the granophyres are 

 petrographically similar to, and sometimes in fact almost indis- 

 tinguishable from, the altered rhyolites of the Upper Acid Series. 1 

 This suggests that the two sets of rocks — granophyres and 

 rhyolites — are closely related as to age. The Upper Acid Series 

 represents the highest volcanic horizon in the district, 3 no igneous, 

 rocks, whether intrusive or extrusive, occurring above this level, 

 nor are there even any distinct ash-bands in the slates above the 

 Upper Acid Series. The absence of any higher volcanic rocks, 

 furnishes an additional argument in favour of the close relationship, 

 in age between the intrusive granophyres and the extrusive 

 rhyolites. 



Now, the age of the rhyolites is known approximately, since the 

 slates immediately overlying the Upper Acid Series contain fossils 

 that suggest a correlation with the Arctus Beds of South Wales, 3, 

 that is, with the lower part of the Upper Dicranoc/raptus Shales, 

 which are low down in the Bala Series. If, consequently, there is. 

 a real connexion between the rhyolites and the granophyres, it 

 follows that the intrusions probably took place in, at the very latest,, 

 quite early Bala time. 



The intrusions of the Dolgelley and Cader-Idris districts thus fall 

 into line as regards their age, as also in their petrographic char- 

 acters and mode of occurrence, with certain intrusions in the Lake 

 District — Buttermere, Carrock Fell, etc. — to which a pre-Upper- 

 Bala, or earlier age, has been ascribed by Mr. J. F. N. Green. 4 



IV. The Tectonics. 



When a district is subjected to severe stress, the usual sequence 

 of events appears to be as follows : — 



(1) Intrusion of hypabyssal rocks. 



(2) Folding-. 



(3) Cleavage. 



(4) Faulting. 



But there is often a certain amount of overlapping as regards, 

 the main events, and thus what one may look upon as the ' normal ' 

 sequence is more or less modified. A further complication may 

 be introduced owing to later movements having followed (post- 

 humously) lines of earlier movement : wherefore faulting, for 

 instance, may belong to two different periods. 



In the Arthog-Dolgelley district both factors — (a) overlapping 

 of the different types of movement, and (b) repetition of the 



1 It is intended to present a detailed account of the petrology and strati- 

 graphy of these higher rocks in a subsequent communication. 



- A. H. Cox & A. K. Wells, op. cit. p. 424. 



:i T. C. Cantrill, H. H. Thomas, & O. T. Jones, 'The Country round 

 Haverfordwest : Geology of the South Wales Coalfield, pt. xi ' Mem. Geol.. 

 Surv. 1914, p. 40. 



4 Proc. Geol. Assoc, vol. xxviii (1917) p. 26. 



