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parent difficulties in connexion with, these old rocks. That 
some of these chlorite and talc schists occur at various other 
horizons in the Pebidian, alternating with clay slates, quartz 
rocks, breccias, and conglomerates, is beyond doubt ; and I 
may mention as spots where these alternations may be well 
observed, the valley of St. David’s, north of the Cathedral, 
Treginnis, near St. David’s, where they occur, as in Anglesey, 
in association with serpentinous limestone, and Amlwch in 
Anglesey. It is certain, therefore, that though the larger por- 
tion of the Anglesey schists may belong to the lower group of the 
Pebidian, they are not restricted to that horizon. This partial 
metamorphism, visible chiefly in rocks most susceptible of 
change, is highly characteristic of the Pebidian formation 
wherever it is exposed in Britain. It is sufficiently marked to 
separate the formation clearly from the succeeding Cambrian ; 
and, on the other hand, it is too imperfect to cause any confusion 
between the majority of the rocks in this formation and the 
more completely crystalline groups belonging to the Dimetian 
or Lewisian. 
The upper group is largely made up of brecciated rocks 
alternating with purplish, reddish, and greenish slates and 
schistose rocks. Indurated volcanic ashes, lava-flows, and bands 
of limestone and serpentine occur in both. It may be well to 
designate these groups by local names, and I propose therefore 
to call the upper the Treginnis group from being largely ex- 
posed at that place, on the shore of Bamsey Sound, near St. 
David’s ; and the lower, the Menai group, after the Menai 
Straits, where they are well displayed. The prevalent strike in 
this formation is from north-east to south-west. 
This formation occurs in many parts of Scotland, especially 
in the Grampians ; and it is also found, according to Dr. Hunt, 
in the north-west of Ireland, about Lough Foyle. It occurs also 
in Charnwood Forest, in Shropshire, in the Malvern Hills, and 
probably also in Cumberland and in Cornwall. The following 
table will explain the supposed order in which the Eozoic 
rocks occur in this country : — 
