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tion to their age, and that the evidences of these changes, which 
could not have affected more recent deposits, are usually suffi- 
ciently well marked to he readily recognized. 
I have described rocks belonging to this formation in Angle- 
sey, Caernarvonshire, and many parts of Scotland ; and Prof. 
Bonney and Dr. Callaway have noticed them in Shropshire. 
The series may be advantageously divided into Upper and 
Lower groups: the Upper represented by the so-called quartz 
schists of Porthlisky, St. David’s, and probably by the thin quart- 
zose gneisses and mica schists of Gairloch and Ben Fyn, Scotland ; 
and the Lower, by the massive granitoid and gneissic rocks of 
Bryn-y-Gfarn, St. David’s, and Twt Hill, Caernarvon, and pro- 
bably also of parts of Ben Fyn and the mountains at the head 
of Loch Shiel in Scotland. I propose for the groups the local 
names Porthlisky and Bryn-y-Garn. 
The prevailing strike in the formation is from N. W. to S. E. 
3. Arvonian. 
This name was given by me in 1878 to a group of rocks, in 
some important particulars unlike the Dimetian, though up to 
that time it had been included in that formation. Subsequent re- 
searches in Horth and South Wales have revealed the fact that this 
is one of the most important of the Pre-Cambrian formations. In 
Pembrokeshire it occupies large areas (see Sect. 2 and 3, p. 308), 
and is there found to consist mainly of highly siliceous rocks, of 
the type called by the Swedish geologists ‘ halleflinta.’ Breccias, 
quartz-felsites, and other quartzo-felspathic rocks, also enter 
largely into its composition. In Caernarvonshire, as I have 
shown in previous papers, many important ridges, such as the 
Bivals, and those to the north and south of Llyn Padarn, and 
between Twt Hill, Caernarvon, and Bangor, consist almost 
entirely of rocks belonging to this formation. In Anglesey, I 
pointed out in 1878 an area of these rocks near Ty Croes. 
This I have since found, in company with Prof. Hughes, 
to extend much farther to the north than I at first sup- 
posed, in fact as far as Bodafon Mountain. At the latter 
place the ‘ halleflintas ’ are perhaps better exposed, and 
more nearly of the type of those in Pembrokeshire, than in 
any other place in Horth Wales.* In my former paper I men- 
tioned, in describing the Pebidian rocks of the west coast of 
Anglesey, that ‘ possibly some, such as the rocks of Holyhead 
Mountain may prove to be of different age and probably older 
* Since I examined these rocks with Prof. Hughes, Prof. Bonney, in an 
appendix to an interesting paper by Dr. Callaway ( Quart. Journ. Geol. Soc. f 
May 1881), has published some notes on these rocks which seem to confirm 
the above view. They are, however, called by him ‘ Quartz-schists.’ In Dr. 
Calloway’s map much too small an area is given to these rocks. 
JBXC 
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