650 ME. C. A. MATLEY ON THE [Aug. 1 899, 



Series, consisting of highly crumpled, green, altered slates, agreeing 

 closely with some seen near Llaufair-y'nghornwy Rectory, about 

 I mile away. At the southern boundary the Ordovician black slates 

 can be seen dipping into the hill, and there appears to be an inver- 

 sion accompanied by a thrust, the emergence of which sweeps round 

 the south of the hill to the western side, where it merges into and 

 may form part of the fault that here too brings the Green Series 

 and the Ordovician ground into juxtaposition. At the south-eastern 

 extremity, in the yard of the cottage called Cefn-du-mawr, the 

 black shales which dip into the hill are interbedded with numerous 

 bands of a green breccia unmistakably made up of fragments 

 of the green altered slates. These fragments, like the parent- 

 rocks, are in the condition of sericitic phyllites, and afford the 

 clearest evidence that the Green Series had not only been exposed 

 to denudation in Middle Ordovician times, but that the green rocks- 

 had even at that period undergone considerable alteration. 



(c) Conclusion. 



Both palaeontology and stratigraphy make it impossible to arrive 

 at any other conclusion than that the Green Series is older than the 

 recognizable Ordovician. The disposition and dip, and even the 

 apparent interstratifications of the rock-groups, are no guide to the 

 true structure of the district, which is comparable in these respects 

 with such regions of powerful earth-movements as the Alps, the 

 Scottish Highlaiids, and other mountain-ranges. The barren green 

 rocks have been brought up and driven southward over black 

 Ordovician and perhaps even Silurian slates by a great thrust which 

 extends from sea to sea in a sweeping curve. This thrust is steeply 

 inclined at its eastern end ; at its western end it show^s the low 

 slope characteristic of the thrust-planes of the North-western 

 Highlands. I have instanced other thrusts of varying hade which 

 also bring the Green Series over the Ordovician, while the latter is 

 elsewhere seen to lie with a strong unconformity upon the green 

 beds and to contain fragments derived from them. 



As to the actual age of the Green Series, all that Northerit 

 Anglesey teaches us at present is that these beds are older than the 

 Llandeilo, and had been subjected to alteration, foliation, and denu- 

 dation before Upper Llandeilo times. Geologists who have worked 

 over the whole of Anglesey find them to agree so closely with rocks 

 in other parts of the island, which they consider pre-Cambrian, that 

 a similar age for the ISTorthern Anglesey rocks must be provisionally 

 conceded. 



IV. Eaeth-movements and their Effects. 



The foregoing account will have made it evident that jSTorthera 

 Anglesey is a region affected by powerful earth-movements that 

 have bent, folded, broken, crushed, contorted, and overturned the 



