ANNIVERSARY ADDRESS OF THE PRi:SIDENT, 1 33 



bands of black shale may also have been l)roiight into their 2)rcseiit 

 positions by faults, and that they do not really belong to the series of 

 strata among which they lie. But this suggestion is completely dis- 

 proved by the coast-sections, which exhibit many thin leaves of black 

 shale sometimes less than an inch thick. These and the ashy 

 layers containing the Orthis and other fossils form an integral part 

 of the so-called " Amlwch slates.'' 



It thus appears that the area coloured " altered Cambrian " on 

 the Survey map, and regarded as pre-Cambrian by some later 

 observers, is proved by the evidence of fossils at its base, towards 

 its centre, and at its top to belong to the Lower-Silurian series, 

 probably to the Bala division. That this was the geological horizon 

 of part at least of the area was recognized by Sir A. llamsay, though 

 he confessed himself unable " precisely to determine on the north 

 coast of Anglesey how much of the strata are of Silurian and how 

 much of Cambrian age " *. Professor Hughes was the first to sug- 

 gest that the whole of these rocks should be referred to the Bala 

 group t. 



I have dwelt on the determination of the true geological age of 

 the rocks of the north of Anglesey because of the diversity of 

 opinion respecting them, and because of their great interest in 

 regard to the history of volcanic action in Wales. These rocks 

 contain a record of volcanic eruptions, probably contemporaneous 

 on the whole with those of the Bala period in Caernarvonshire, yet 

 independent of them and belonging to a diflferent type of volcanic 

 energy. The vents lay in the northern half of Anglesey, where some 

 of them can still be seen. The materials ejected from them were, 

 so far as we know, entirely of a fragmentary kind. Vast quantities 

 of detritus from the hugest blocks to the finest dust were thrown out ; 

 but though dykes rose up and filled fissures in the vents, no trace has 

 yet been found of the outflow of any lava. In the lower part of 

 this volcanic series the bedded breccias are remarkably coarse. 

 Their included stones, ranging up to six inches or more in diameter, 

 are usually more or less angular, and consist mainly of various 

 felsites. Layers of more rounded pebbles occasionally occur, while 

 the bedding is still further indicated by finer and coarser bands, and 

 even by intercalations of fine tuffs and ashy shales. Towards their 

 upper limits some of these volcanic bands shade off into pale grey or 

 greenish ashy shale followed by black sandy shale of the usual kind. 



* Mem. Geol. Surv. vol. iii. 2nd ed. p. 242. 



t Proc. Camb. Phil. Soc. vol. iii. (1880) pp. 341-3-18. 



