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Vol. 66.] TREMADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 173 



facies which we call Glenkiln. 1 It is noteworthy that from the 

 time of Sedgwick to that of Cole 2 arjd Jennings & Williams, 3 slates 

 with pisolitic iron-ore have been regarded as the most characteristic 

 of the ' Tremadoc ' Slates, and rocks at Llanelhaiarn, Abersoch, 

 Cader Idris, etc., are identified as of Tremadoc age, because they 

 are rather similar to these Llandeilo rocks. 



Above the belt of pisolitic iron-bearing rocks the slates lose 

 their dark colour, and pass into blue or greeuish-grey banded slates, 

 with coarser flaggy or ashy beds above them. Fossils in the higher 

 rocks are difficult to find, and I have been compelled to map the 

 area on lithological evidence alone. Upon the map I distinguish 

 the following four very distinct lithological types : — 



(1) The well-known gabbroid dolerites, 4 which form the crags of Y Gesail 



Y Castell above Tremadoc, and Pant-ifan behind Tan-yr-allf. 



(2) Banded blue-grey slates, into which the dolerites are intruded, 



and which they, when in contact with them, have altered into 

 ' spilositic' flags and hornstones. 5 



(3) Ash-bands, usually of andesitic composition, and varying in their 



texture from fine tuffs to agglomerates. 



(4) A non-porphyritic vesicular rock, probably once an andesitic lava, 



which underlies, and is closely associated with, the most prominent 

 ash-bands and agglomerate. 



Regarding the slate-rock (2) as a matrix in which the others are 

 set, it is apparent that on the map the dolerites (1), the ashes (3), 

 and the vesicular andesites (4) alike appear as strings of elon- 

 gate lenticles. Since the time of J. E. Davis 6 these dolerites have 

 been recognized as intrusive, but, unless we postulate a separate 

 volcanic vent for each lenticle of ash or agglomerate, the stringing- 

 out of the andesite exposures (4) can only be explained by faulting; 

 and since we have proved such faulting along the line of the 

 pisolitic-iron belt, it is not unreasonable to explain the further 

 repetitions and general 'shuffled' arrangement of the whole country 

 to the north-east of it upon a similar plan. 



The Andesitic Volcanic Rocks. 



The main outcrops of the andesitic rocks (3) and (4) form sub- 

 sidiary cliffs on the scarp-faces of the hill-masses which are 

 determined by the dolerites. They occur in four main series cf 

 exposures. The westernmost exposure of these is on the south- 

 western face or edge of Craig-y-gesail (fig. 1, p. 168). This exposure 

 shows only a few feet of cherty ash, which is too much baked by the 



1 C. Lapworth, Quart. Journ. Geol. Soc. vol. xxxiv (1878) p. 253. 



2 G. A. J. Cole & V. Jennings, ibid. vol. xlv (1889) p. 436. 



3 V. Jennings & G. J. Williams, ibid. vol. xlvii (1891) p. 372. 



4 A. Harker, ' Bala Volcanic Series of Caernarvonshire ' (Sedgwick Prize 

 Essay) 1889, § vi. 



5 J. J. H. Teall, ' British Petrography ' 1888, p. 219. 



6 Quart. Journ. Geol. Soc. vol. ii (1846) p. 70. 



