Vol. 55.] SERPENTINE AND ASSOCIATED ROCKS IN ANGLESEY. 301 



the marginal parts, or where more irregular and narrower intrusive- 

 streaks, may have given way.^ The smaller outcrops or less 

 important varieties of rock (diallage-rock, etc.) have either yielded^ 

 or resisted in correspondence with the effects produced on the mass 

 into which they intrude ; but local crushing is often marked, as in 

 the actinolitic rocks already described. 



Thus all the igneous intrusions of the district, except the dykes, 

 are more or less affected by pressure, and so are anterior to it. 

 They, however, are clearly not of one age, although they may 

 not be separated by any long intervals of time. The order of 

 succession seems to be : — 



(1) Peridotite (the original of the serpentine), into which the 



diallage-rock and the enstatite-rock very soon intruded. 



(2) Gabbro. 



(3) The original of the chlorite-rock. 



(4) Diabase or basalt-dykes not modified by the pressure which 



affects all the others. Some of these dykes may be of 

 Carboniferous or a later age. 



As for the date of the structure, though there were some post-- 

 Carboniferous movements producing faulting, the great disturbance 

 which affected so many rocks in Anglesey and Caernarvonshire was 

 post-Bala and pre-Silurian."^ The action of this in Caernarvonshire 

 has been discussed by Mr. Harker,'^ who thinks that the thrust 

 there came chiefly from the south-east. In Holyhead Island and 

 adjacent parts of Anglesey the general dip of the schistose planes is 

 towards a northerly point, as if the thrust had acted from that 

 direction. In the gabbro and the serpentine * this dip is usually at a 

 low angle, but it varies and becomes higher in places, probably 

 near faults. 



Beyond the fact that the peridotite must bepre-Silurian, we have 

 very little clue to its date. The rocks which it apparently cuts 

 are probably Archsean, and the only two in the district petrologically 

 related to it are the picrite of Caemawr and that of Pengorph- 

 wysfa, the former of which, according to Prof. Hughes, cuts Ordo- 

 vician (Arenig) beds.' One thing, however, is certain, that the more 

 ancient rock-masses in this and other portions of Anglesey have 

 undergone extreme pressure, and are comparable with those in parts 

 of the Scottish Highlands and of the Alps : in other words, that 

 what are now lowlands may represent the roots of a former 

 mountain-chain. 



^ A man quarrying one of the large bosses stated that the rock became 

 harder as he got farther in. 



^ There was, however, an earlier (Archaean) set of earth-movements, for in 

 the basal Cambrian conglomerate are found pebbles composed of schist or 

 gneiss, which had already acquired a pressure-structure. 



^ ' Bala Volcanic Series of Caernarvonshire,' 1889, p. 113. 



* We think, however, that, as mentioned on p. 289, this rock is locally 

 affected by a post-Carboniferous movement. 



° T. Gr. Bonney, ' On the so-called Diorite of Little Knott, with further 

 Eemarks on the Occurrence of Picrites in Wales,' Quart. Journ. Greol. Soc, 

 vol. xli (1885) p. 515. 



