HARLECH AND LONGMYND SERIES. 55 



beds consist of grits, conglomerate, mudstone, and slate. The 

 conglomerate contains pebbles of quartz, quartzite, lydian stone, 

 jasper, mica-schist, etc., and rests unconformably on the Archcean 

 rocks, from which the fragments have been derived. The general 

 succession of Cambrian rocks in this neighbourhood appears to be 

 as follows^ (see Fig. 8, and Fig. 5, p. 42): — 



3. Black and purple Slates. 



2. Brown Sandstones, often fossiliferous. 



I. Basement-conglomerate series, consisting sometimes of a quartz con- 

 glomerate, sometimes of a felsite conglomerate, and sometimes of 

 quartzose or felspathic grits : varying according to the character of 

 that part of the underlying series on which it rests. 



The Quartz-conglomerate of Twt Hill, Caernarvon, about which 

 much controversy has been raised, and which is seen in juxta- 

 position to the Archaean granitoid rocks, has been shown by Prof. 

 Hughes to form the base of the Cambrian series.^ Symptoms of 

 unconformity were noted by Mr. G. Maw in the lowest Cambrian 

 Beds at Llyn Padarn, near Llanberis. This irregularity is (in the 

 opinion of Prof. Hughes) due to lateral pressure acting on beds of 

 unequal texture and character. The coarser and more sandy beds 

 were crumpled and protruded into the finer beds, which were com- 

 pressed and cleaved.^ 



Anglesey. — The exact equivalents of all the older rocks of this 

 Island have not yet been definitely determined. Two slate groups 

 have been described by Prof. Hughes, the upper belonging to the 

 Silurian system, and a lower in which Arenig, Tremadoc, and even 

 older beds may be recognized. Below are the basement beds 

 of the Cambrian, consisting of conglomerates, grits, and sandstones.* 

 The Baron Hill grit near Beaumaris probably belongs to the 

 basement conglomerate. 



The Clymwr and Llanfihangel or Treiorwerth conglomerates, 

 with intervening Cwaen shales, have been described by Dr. Callaway 

 — they fringe the granitoid and slaty rock (Archaean) of Tywyn ; 

 and are overlaid by grits with Orthis Carausii and other fossils 

 of Upper Cambrian or Ordovician age. He doubts whether any 

 Lower Cambrian rocks are present in Anglesey.^ (See also p. 43.) 



Caerfai and Solva Beds. 



In South Wales two divisions have been made in the Harlech 

 series by Dr. Hicks (1881), and named respectively the Solva and 

 Caerfai groups, from localities near St. Davids, in Pembrokeshire, 



1 Q. J. XXXV. 688 ; and R. D. Roberts, G. Mag. 1882, pp. 114, 152. 



^ G. Mag. 1 88 1, pp. 194, 439. See also papers by T. G. Bonney and F. T. S. 

 Houghton, Q. J. xxxv. 309, 321 ; G. Mag. 18S2, p. 21 ; Q. J. xxxix. 478; and 

 H. Hicks, Q. J. xl. 197. 



* Q. J. xxxiv. 143, 764. 



* Q. J. xxxviii. 27 ; see also Proc. Cambridge Phil. Soc. iii. 341. 

 5 Q.J. xl. pp. 572, 581. 



