E. Greenly — Age of Later Anrjlesey Dyhes. 161 



north-east siJe of tlie road is seen only in the cutting, but that on 

 the south-west side (which 1 will call the Holland Anns dyke) 

 has been traced to the E.S.E. for a distance of more than a quarter 

 of a mile, retaining all its peculiar cliaracters. In this short 

 distance it passes from the red rocks of the cutting across a faulted 

 belt of Carboniferous Limestone, then across a narrow belt of 

 Ordovician shales, and from these into mica-schists belonging to 

 the old complex. It traverses, indeed, nearly all the great rock 

 groups of Anglesey, and passes, unaffected so far as can be seen, 

 across two, if not three, main faults. Moreover, the two faults in 

 question must be at least of Post-Carboniferous, and if Sir A. C. 

 Kamsay's view be correct, of Post-Permian age. 



It appears, therefore, that the dykes must be of much later date 

 than has been hitherto supposed. The negative evidence of the old 

 pit- workings, that they belong to a period between Carboniferous and 

 Permian, loses its value in face of the positive evidence from the 

 railway cutting,' and certainly no volcanic episode of that date is 

 known elsewhere in Britain. It is conceivable, indeed, that they 

 might belong to the Permian period itself, but this is very unlikely, 

 even should the red rocks prove to be Carboniferous. No such 

 regular system of N.W.-S.E. dykes is known in the Permian 

 volcanic groups, and in any case we should hardly expect Permian 

 dykes to pass across main faults of Post-Carboniferous date. Now 

 the next period of volcanic activity within the British area is that of 

 Tertiary time (Geikie, " Ancient Volcanoes of Great Britain," vol. ii, 

 p. 107) ; and to that period, therefore, these dykes most probably 

 belong. 



In addition to the stratigraphical considerations above set forth, 

 there are others that lend supjjort to this view. 



The petrological characters of the dykes are on the whole 

 favourable to it rather than otherwise. While the majority are 

 somewhat sub-basic dolerites, with andesitic selvages, there are 

 a certain number of typical ophitic olivine-dolerites, of which the 

 Holland Arms dyke is one. I have compared a slide of it and of one 

 of the less basic dykes a short distance away, on the south side of 

 the Holyhead Eoad, near the old Jerusalem Inn, with a number of 

 slides of Tertiary dykes in the Jermyn Street collection, and there is 

 no difficulty in matching them. The Holland Arms dyke itself 

 resembles a dyke from Goat Island, Kaasay, to mention a particular 

 instance. 



Mr. Harker, who has described these dykes (Geol. Mag., 1887, 

 p. 409 ; 1888, p. 2G7 ; also '■ Bala Vole. Series Carnarv.," pp. 106-109, 

 and "Petrology for Students," 2nd ed., p. 129), and who has had 

 exceptional opportunities for studying the Tertiary dykes of Skye, 

 writes me : — " Petrographical analogies would seem rather to 

 support than to debar such a conclusion [viz., as to their Tertiary 

 «(/e]. The Menai Straits dykes are for the most part andesitic 

 dolerites and augite-andesites, not rocks of very basic composition, 



' It may be remarked in passing that the statements as to certain faults in the 

 coalfield, if true, lose none of their importance as evidence. 



DECADE lY. VOL. VII. — XO. IV. H 



