Vol. 55.] GEOLOGY OF NORTHERN ANGLESEY. 667 



integrity of the strata would be the results of two systems of 

 movement acting somewhat obliquely to each other, and this may 

 have been the case in Northern Anglesey. Jointing, cleavage, and 

 bedding would all be of service in producing a cataclastic structure. 



(g) Age of the Movements. 



The thrusting of older beds over the Ordovician rocks and the 

 disruption of the Ordovician in Forth Newydd and Forth y Corwgl 

 determine the age of the last period of great movement in 

 Northern Anglesey as post-Ordovician. This movement is 

 older than the dykes that cut through the crush-conglomerates, but 

 we are not sure whether they are of Carboniferous or of Tertiary 

 age. The Carboniferous rocks of Anglesey being, however, compara- 

 tively undisturbed, as also seem to be the underlying deposits mapped 

 as Old Fed Sandstone, the period of the movement must lie between 

 Upper Ordovician and Upper Old Fed times. This movement may- 

 well have been contemporaneous with that of post-Cambrian and pre- 

 CarboDiferous date which produced disruption in the Manx slates.^ 



The recognizable Ordovician rocks have a less altered and, as a 

 rule, a less disturbed look than most of the older deposits, and the 

 Ordovician conglomerates give evidence that the older rocks had 

 undergone considerable alteration, movement, and upheaval before 

 Llandeilo times. 



V. The Northern Complex and its Relation to the Ordovician. 



We now return to a consideration of the highly-complicated area 

 spoken of as the Northern Complex. Apart from Famsay's 

 description,^ the only account which gives any detail of this area 

 appears to be that of Frof . Blake under the head of ' The Disturbed 

 Volcanic Group.' ^ We have seen that the ' agglomerates ' of the 

 latter writer's description are crush-conglomerates, and that they 

 are, in the main, disrupted portions of his ' Sedimentary Series ' 

 (Green Series). The limestones, or at least some of them, he con- 

 sidered to be the result of ' the action of calcareous springs,' ^ while 

 the ' quartz-knobs ' he looked upon as tlie product of ' pre-Cambrian 

 geysers.'^ This group abounds in other lirhological types, and in the 

 complex lie the conglomerates, sandstones, and shales whose age is 

 proved by the fossils recorded above (p. 640) to be Llandeilo. 

 Famsay ® believed that rocks of two systems, namely, Lower Silu- 

 rian (Ordovician) and 'altered Cambrian,' occurred in this part 

 of the district, but he found it impossible to determine how much 

 of the strata belonged to each system, though he was inclined 

 to consider the Cemaes limestones and all their associated rocks as 



^ Lamplugh, ' Crush-conglomerates of L of Man,' Quart. Journ. GeoL 

 Soc. vol. li (1895) p. 584. 



2 Mem. Geol. Surv. • Geol. N. Wales,' 2nd ed. (1881) p. 240. 



3 Quart. Journ. Geol. Soc. vol. xliv (1888) p. 517. 



* Ibid. p. 519. 5 p^^ ^ ^r^Q 



^ Op. supra cit. p. 242. 



