GEOLOGY OF ANGLESEY. 



229 



The series is probably complete in 3 ; but in running the section I 

 could get no exposure between an obscure opening of hiilleflinta and 

 the grey gneiss, and I had not time to search beyond the highroad. 



b. Slaty Series. 



This group occupies four areas, viz. the small mass south- 

 west of Pentraeth, the east side of the Llangefni "syncline," northern 

 Anglesey, and the western division north of the Porth-y-defaid 

 fault. The chief facts are thus summarized : — 



(1) South ivesi of Pentraeth (Wugan-bach ridge). 



Slaty and dolomitic rocks. 



(2) Llangefni district. 



Slates (some hypometamorphic *), grits, green conglomerates, 

 purple and green shales, hornstone, calcareous beds. 



(3) Northern district. 



Chloritic schists, altered grits, green and purple shales, slates 

 (some hypometamorphic), quartz-conglomerates, limestones, 

 hornstone. 



(4) No rth-iv ester n district. 



Yellow felspathic shales and breccias predominating, quartz- 

 conglomerates, pale-green slates, and limestones. 



I am not prepared to submit a scheme of the succession. The rocks 

 of area 1 are not unlike some of those near Cerrig Ceinwen in area 

 2. Comparing 2 and 3, the rocks are similar as a whole ; but the 

 green conglomerate, so conspicuous in 2, is not seen in 3, while the 

 quartz-conglomerates and grey limestone of 3 do not appear in 2. 

 Portions of 3 are also altered to schist, while in 2 the transition 

 into foliated rock is rarely complete. The felspathic breccias of 4 

 do not certainly occur in any of the other areas ; but its quartz- 

 conglomerates, slates, and limestones are similar to those of 3. The 

 interesting Ehosbeirio shales in 3 have less complete representatives 

 in 2, but are fully exhibited in 4. 



The rocks of these four areas, though geographically isolated, are 

 closely connected by their mineral and penological characters, and 

 I have no hesitation in placing them all in the same group. Should 

 the chloritic schists of the north be separated from the newer series, 

 it will not materially affect my general conclusions. The same 

 remark applies to the small patch of gneissic rock at Bryn 'r Eglwys. 



II. Evidence of Age. 



a. Relations to Palaeozoic Groups. 



Gneissic Series. — It is not necessary to recapitulate the evidence 

 for Precambrian age. The sections in the quarries near Nebo would 

 alone be sufficient to remove' all doubt. 



Slaty Series. — These rocks do not clearly underlie any part of the 

 Cambrian ; they are, indeed, overlain by a purple conglomerate on 

 the north coast, west of Tor llwyn. The pebbles of this rock are a 



* The term " submetamorphic," used by some authors, is objectionable on 

 grammatical grounds. 



Q. J. G. S. No. 146. b 



