230 



C. CALLAWAY OX THE AKCHiEAN" 



purple quartzo-felspathic grit : "but I have no certain proof of its 

 age. 



The Precambrian age of the series may be inferred from included 

 fragments. The rocks west of the granitoid zone are shown by the 

 fossils collected by Prof. Hughes to be Tremadoc and Arenig. In 

 the conglomerates west of Llanfaelog, which appear to be of at 

 least equal antiquity, a large proportion of the pebbles are certainly 

 derived from the Slaty series, pale green and purple hornstone and 

 slate being the most common. It is also worthy of notice that 

 the Harlech conglomerates of Caernarvonshire contain fragments 

 of green and purple slate *, which must therefore be Precambrian: 

 and of no distant source of derivation. 



The relations of these series to the Cambrian rocks in contact 

 indicate the same result. At numerous points the two groups are 

 brought together by faults ; but while the Slaty series is generally 

 more or less altered, the Cambrian shales are quite unchanged, and 

 I have never observed the slightest indications of a passage between 

 the two, the line of demarcation being always clear and sharp. The 

 Slaty series must therefore have undergone partial metamorphism 

 before the Cambrian period, otherwise it is difficult to see how the 

 Cambrian rocks could have remained unaltered. 



The black shales in some localities dip as if they would pass under 

 the altered scries ; but at many points the latter also appear to pass 

 beneath the former. In such a shattered district this evidence is 

 worth nothing in either case, unless an ' unbroken sequence can be 

 proved. 



I am not prepared absolutely to deny that true Harlech rocks 

 occur in Anglesey. There arc beds, especially near Llangristiolus, 

 which lithologically do not appear to be very distinctively Pre- 

 cambrian ; but, as stratigraphically they are closely associated with 

 the Slaty series, it is safer to place them in that group. 



b. Relations to each other. 



In all parts of Anglesey the junction of the two series, whenever 

 they are in contact, is a fault. In the east and centre they generally 

 strike in the same direction ; but in the north-east (near Paris 

 Mountain), at the base of Bodafon Mountain, and in the western 

 district, as seen north and south of the Porth-y-defaid fault, the 

 strikes are discordant, sometimes approaching a right angle. The 

 much more intense metamorphism of the gneissic series would seem 

 to point to a greater antiquity. Some additional light is thrown 

 upon the subject by the microscope. Prof. Bonney, in note 47 (p. 234), 

 already noticed, is decidedly of opinion that the Llanfechell grit, a 

 characteristic variety of the newer rocks, is derived from the older 

 series. 



c. Relations to other Areas. 



(1) Caernarvonshire. 

 Comparing the Slaty series with the Bangor group, the re- 

 * Bonney, Quart. Journ. Geol. Soc. vol. xxxv. p. 311. 



