476 PROF , T, Q. BONNET ON A SECTION 



unaltered, as when the temperature of one part has been locally 

 elevated by the proximity of large masses of igneous rock, or when 

 it has been exposed to very exceptional pressure in the presence of 

 water at a slightly higher temperature than usual ; but here there 

 is no reason for suspecting any agent of change to have been more 

 intense than usual. Further, at Garth Ferry we have Ordovician 

 rocks * quite unaltered ; and on the Anglesey shore, on either side of 

 the landing-place, is a quartz grit, faulted down against the schists, 

 entangled with which may be seen some remnants of a slaty rock 

 similar to that on the mainland. This appears to me to be rightly 

 mapped by the Survey as Ordovician, and I should correlate it with 

 the quartz grit and conglomerate near th© opening of the sewer at 

 Garth, which, as it seems tome, occurs there in association with th© 

 dark slaty beds, and not with any part of the Cambrian conglomerate. 

 As, then, in the immediate neighbourhood, we find Ordovician 

 rocks and even strata probably rather below the Cambrian practically 

 unaltered, we cannot hesitate to consider these Menai schists* 

 Archsean, and should not be disposed to refer them to the newest 

 period in this great group. But these Menai schists have a general 

 resemblance to the bedded schists which abound on Holyhead Island 

 (as already described by myself and others) and in the intervening 

 districts of Anglesey. I think no petrologist could hesitate, in the 

 absence of any evidence of special local metamorphism, to refer all 

 the above roughly to one group. It would therefore seem vain to 

 speculate on any "gnarled rocks," identical in constitution with these, 

 being by any possibility of Ordovician or even of Cambrian age. It 

 might perhaps happen that a small patch of one of the older Palaeo- 

 zoics, locally nipped and tremendously squeezed, should exhibit a 

 "universally slickensided " appearance, which would cause some 

 macroscopic similarity to these true schists ; but I doubt whether, 

 even then, there would be much real correspondence when they were 

 examined microscopically. Here, at Baron Hill, we have almost side 

 by side true schists and rock which all must admit to be at least 

 older than the greater part of the Cambrian ; yet, as we have seen, 

 the latter is almost unaltered. 



We may, then, I think, safely regard these micaceous and chloritic 

 schists of Anglesey as Archaean. By some authors they have been 

 claimed as Pebidian. Here, however, the question arises, What is 

 Pebidian ? If we take as its type the series on which the name 

 was first conferred, the group beneath the Cambrian conglomerate 

 at St. David's, we may at once repudiate the identification f . That, 

 like the rocks of Charnwood Forest, is a group largely consisting of 

 volcanic materials and comparatively slightly altered. To such a 

 group we may assign the series in Caernarvonshire between the Cam- 

 brian conglomerate and the great rhyolitic masses (which, indeed, I 



* Probably of about Llandeilo age (see Mem. Geol. Surrey, vol. iii. ch. xxii.). 



•j- This paper was written prior to the reading of Dr. A. Greikie's paper " On 

 the supposed Pre-Carubrian Rocks of St. David's ; " but, notwithstanding the 

 efforts of the Director General to efface the limit between Cambrian and Pebi- 

 dian, the author sees no reason to alter any thing that he has written here. ' 



