C. CALLAWAY ON THE ROCKS OF ANGLESEY. 567 
39. The ArcHzan and Lower Patmozotc Rocks of Aneresty. By 
C. Cattaway, Esq., D.Sc., F.G.S. With an Apprnprx on the 
Prtrrotoey of the Rocxs by Prof. Bonnry, D.Sc., F.R.S., Pres. 
G.S. (Read May 28, 1884.) 
In papers communicated to this Society * and the Geological Maga- 
zine t I have described in some detail the Archzean formations of 
Anglesey. I came to the conclusion that there was no satisfectory 
evidence of more than two of these systems, the Gneissic and the 
Slaty—the former metamorphic, the latter, on the whole, hypo- 
metamorphic, but passing here and there into true schists, hardly 
less altered than some of those which occur in the older series. 
This work was too laborious and absorbing to permit me to pay 
minute attention to the Paleozoic strata of the island; and this was 
the less necessary, since other workers were approaching the inyes- 
tigation from the side of the newer rocks. Their labours tended 
strongly to support my views, so far as they referred to the central 
part of the island ; but much remained to be done in the north and 
west. I therefore determined to work round the margins of the 
Archean areas on which Holyhead and Amlwch are respectively 
situated, and I have had the satisfaction of ascertaining that these 
masses are fringed on the east and south respectively by shore- 
deposits, which furnish evidence similar to that + which has been for 
some years familiar to us in the conglomerates which overlie the 
central axis. I have also discovered most interesting proof of the 
relations between the two Archeean systems. 
A. RELATIONS BETWEEN THE AMiwcH ARCHAHAN MASS AND THE 
Patzozoic Rocks to THE SovurTH, 
_ As the Paleozoic strata of Northern Anglesey dip in a northerly 
direction, in apparent conformity with the altered § rocks which lie 
to the north, the two groups might seem to be in true sequence. 
Sir A. C. Ramsay, however, held || that they were separated by a 
fault; and of the correctness of this view I have no doubt. Sec- 
tions showing the actual contact are very rare. By far the most 
distinct junction is at Porth-y-corwg, on the east coast, where the 
rocks are exposed in a perpendicular cliff. Conglomerate and dark 
shale are brought against the hypometamorphic slates by a fault, 
which is nearly or quite vertical. A notch is formed in the coast 
at the contact of the soft Paleozoic rocks with the tough green 
* Quart. Journ. Geol. Soc. vol. xxxvii. p. 210. 
t Dec. 2, vol. vii. p. 117 (March 1880). 
{ Recently given in greater detail by Dr. Hicks and Prof. Bonney (Quart. 
Journ. Geol. Soe. vol. xl. pp. 187, 200). 
§ Throughout this paper I use the term “altered” as inclusive of “ meta- 
morphic” and “ hypometamorphic.” 
| ‘Geology of North Wales,’ 2nd ed. p. 235. 
