﻿Vol. 66.~\ TKEMADOC SLATES OF SOUTH-EAST CAENARVONSHIRE. 147 



among the lists of Arenig species. This mistake was recognized by 

 Mr. Hopkinson & Prof. Lap worth in 1874, 1 and is pointedly referred 

 to by the latter in his ' Distribution of the Rhabdophora,' 2 but 

 was allowed to pass uncorrected into the second (1881) edition of 

 Earns ay's great work. 



The papers by Hicks fully establishing the faunistic classification- 

 scheme were published in 1873 and 1875 3 ; and, in addition to their 

 incidental application of the new terms to the naming of the rock- 

 series within our district, they contain the suggestion that the 

 Upper Tremadoc Slates (as the term was employed by Salter) are the 

 equivalents of, and are to be included within, his (Hicks's) Lower 

 Arenig. Such a suggestion, involving, as it does, the placing of the 

 Tremadoc as the lowest member of the Ordovician System, is still a 

 matter of interest ; but, as it may involve the complete absorption of 

 the ancient term ' Tremadoc Slates,' it cannot, I think, be proceeded 

 with on the evidence before us. 



The' scheme of classification of the rocks that I have found most 

 useful in working out the rocks of the district is an amplification of 

 the one which I adopted in describing the corresponding rock series 

 at Arenig, 4 and, like the older schemes of Sedgwick and Salter, is 

 based upon a combination of the palseontological and lithological 

 evidence-locally obtained. It is as follows : — 



evidenc 



Caradoc 

 Series. 



Llaxdeilo 

 Series. 



Arenig 

 Series. 



Western or Criccieth District. 



Rbyoiitic ashes and agglomerates. 



Variable dark-grey and black 

 sbivery slates, banded, but with 

 no distinguishable horizons. 



Dark banded slates 

 with intense cleavage ; 

 no fossils found. 



Earthy slates, with occasional 

 ' tuning-fork ' graptolites. 



Shivery slates passing down into 

 Flaggy grits yielding Calymene 



parvifrons. 

 Basal conglomeratic grit. 



Eastern or Tremadoc District. 



Grey slate series, often strongly 

 banded, with a few shelly 

 fossils (Trinucleus and 

 Orthis) in the upper part. 



Andesitic ashes and ashy shales. 

 Vesicular andesites. 



Blue-black slates containing 

 graptolites. Zone of Nema- 

 graptus gracilis. 



Conglomeratic grit of 

 Towyn. 



Ym 



Unconformity. 



1 Quart. Journ. Geol. Soc. vol. xxxi (1875) pp. 631 et seqq. 



2 Ann. & Mag. Nat. Hist. ser. 5, vol. iv (1879) p. 340. 



3 Quart. Journ. Geol. Soc. vol. xxix, p. 39, & vol. xxxi, p. 167. 



4 Ibid. vol. lxi (1905) pp. 612 et seqq. 



