188 EEPORT OF THE AMERICAN COMMITTEE. 
distinct and younger than the Hebridian. The gneissic rocks of 
Anglesey and some of the newer gneiss of the Highlands may 
belong here, (c.) Pebidian, a well-marked group recognizable at 
St. David's, in the Malvern Hills, and in the counties of Shrop- 
shire, Caernarvonshire, Anglesey, Dublin and Wexford. [The 
Arvonian of Hicks is, in some places, a part of the Pebidian, 
and in others of an older gneissic group.] 
Dr. Hicks describes his classification of the Pre-Cambrian 
(Archean, or Eozoic) rocks as Dimetian, the oldest known in 
Wales, having a granitoid character, and consisting of quartz, 
feldspar, and a chloritic mineral; or a schistose character, where 
(as in Anglesey) mica is also found in them.. In other places 
they consist almost entirely of quartz. The Arvonian consist of 
compact highly qnartzose rocks of the halleflinta type, of felsi- 
quartzites, and of rhyolites and acid breccias. In some places 
they are unconformable and in others apparently conformable, as 
the Dimetian. The Pebidian consists mainly of rocks of volcanic 
origin, alternating with schistose, micaceous, chloritic and talcose 
rocks. Instead of acid rocks being, as ia the Dimetian and 
Arvonian groups, the prevailing types^ the basic rocks predomi- 
nate. Agglomerates and breccias occur in great thicknesses. 
Serpentinous and dolomitic rocks are also found at various hori- 
zons. It is a group of great thickness and appears to be uncon- 
formable to the Arvonian. It is covered everywhere in Wales 
by the newer rocks, and the lowest Cambrian conglomerates are 
almost entirely made up of the waste of this and of the preceding 
groups. 
In Scotland the Archean rocks may be conveniently divided 
into at least four groups. The lowest in stratigraphical position^ 
as far as I have examined them, is the Loch Maree group, and 
it consists of the massive hornblendic and the granitic gneisses 
found in many places along the west coast of Sutherland and 
Ross. Next in position I would place the massive quartzose and 
granitoid gneisses of Poolewe and of Loch Shiel of the more cen- 
tral Highlands (the Loch Shiel group). In many respects these 
may be compared with the Dimetian of Wales. The next group, 
according to my views, would include the Glas Bheinn (rather 
massive epidotic and chloritic gneisses, hornblendic rocks and 
black mica schists), and the Ben Fyn series. The latter consist 
of coarse and fine-grained quartzose gneisses, silvery mica schists, 
