KECENTLY EXPOSED NEAK BEATJMAKIS. 477 



should include with the overlying beds). With some part of this 

 the felsitic grits at Baron Hill probably correspond. 



The term Pebidian, then, should designate a comparatively un- 

 altered series (hypometamorphic, as it has been called by Dr. 

 Callaway) which does not appear to be much more sharply marked 

 off from the Cambrian than the Ordovician from the Silurian. It is 

 especially characterized by an abundance of volcanic material, chiefly 

 of an acid type. So far as we can conjecture, it appears to have 

 commenced (I speak, of course, only of Britain) by an epoch of vol- 

 canic activity, when, from orifices opened probably on an old land 

 surface of the more ancient Archaean rocks, flows of glassy lava and 

 great masses of trachytic scoria were discharged — and to have been 

 followed by gradual subsidence, which probably became still more 

 general in the Cambrian period. 



[Note, June 26, 1883. — Since this paper was written I have 

 received, through the kindness of Dr. J. W. Dawson, E.R.S., a very 

 interesting series of small specimens of Huronian rocks from 

 Canada. I have not yet had time to examine the microscopic 

 structure of these;' but macroscopically the resemblance of not a 

 few of them to certain of the Pebidians of St. David's and some of 

 the rocks from Cbarnwood is most remarkable. They are, how- 

 ever, wholly different from the true schists described in the above 

 paper.] 



These Anglesey schists (which present considerable resemblance 

 to certain rocks recently described by myself from the Lizard district, 

 and could perhaps be paralleled by others from the older part of the 

 Pietra Verde group of the Alps) must be regarded as distinctly older 

 than the typical Pebidian. The correlation of them with the last- 

 named group was due, I think, to two misconceptions — an erroneous 

 identification of an Arvonian series in this part of North Wales, and 

 an overestimate of the amount of metamorphism in the St. David's 

 Pebidians. As regards this correlation I see no escape from the 

 following dilemma. If there is Arvonian at Ty Croes (Anglesey), 

 then the schists of Holyhead Island and the Menai must either be 

 much older than the Pebidian or the term must cease to have any 

 classificatory value, as it would include rocks so very dissimilar in 

 their amount of metamorphism. 



In the above remarks I have not attempted to correlate precisely 

 the Baron-Hill grit with the beds on the mainland. All that I 

 maintain is, that it cannot be newer than the Cambrian Conglome- 

 rate of Professor Hughes or older than the great flows of rhyolite. 

 Any thing more than this would involve a digression into the strati- 

 graphy of the beds near Bangor, which I reserve for a separate 

 paper. I may, however, observe that the Baron-Hill beds differ 

 considerably both from the conglomerate which, for some distance, 

 fringes the opposite side of the Menai Strait and runs inland to the 

 east entrance of the tunnel west of Bangor Station, and from that 

 on the top of the hill above the eastern tunnel, both of which are 

 referred by Professor Hughes to the base of the Cambrian series. 



(For the Discussion on this paper, see p. 485.) 



