Vol. 69.] GEOLOGY Oe'baRD3EY ISLAND. 525 



IV. Post-Movement Intrusions. 



Three olivine-dolerite dykes with a north-westerly to south- 

 easterly trend have been noted in the course of the mapping. 

 They are quite uncrushed, and occupy fissures which cut across 

 the structures of the surrounding strata. One of them, some 

 40 feet or more wide, has been excavated by the sea to form the 

 little harbour at Cat'n Enlli. Another but narrower dyke is seen 

 in the cliff-fissure of Ogof y Gaseg : it splits into two branches, 

 both of which die out before reaching the top of the cliff. .The 

 third dyke, 9 feet wide, occupies the fissure-indentation in the 

 ooast at Ogof Hir. In addition, I have detected two small bosses 

 of similar dolerite on the western slope of Mynydd Enlli. 



On the shore at Briw Gerig, east of Mynydd Enlli, as seen from 

 a boat, are many blocks of a basaltic rock which may be a thick 

 dolerite-dyke ; on the other hand, it may be one of the older 

 diabases. 



V. Stratigraphical Summary. 



From the foregoing description it will be seen that the rocks of 

 Bardsey have been shattered by earth-movements, which have acted 

 from directions between north and west. So great is the shattering 

 that almost everywhere the harder bands, whether thick or thin, 

 that were interbedded in the slates have been torn to pieces, and 

 lie as detached blocks and lenticles in the softer slaty strata. 

 The general tectonic arrangement, though to a great extent masked 

 by this local shattering, has been shown to consist of a number of 

 broken overfolds, which are accompanied by thrusts. The rocks 

 themselves are mainly gritty schistose slates, with many thin and 

 some thick bands of grit, quartzite, and limestone. They contain 

 an horizon of variolitic lava and tufaceous shale which indicates 

 that a volcanic episode occurred during their formation. Except 

 in this last-mentioned zone, the bedded rocks seem to be ordinary 

 sediments ; some pyroclastic fragments probably occur in them, but 

 they are not conspicuously present. There are, however, sills of 

 albite-diabase folded in with the sedimentary strata, as also one or 

 more sills of a crushed granite. 



From the nature of the brecciation and the comparatively small 

 amount of mineral change which the beds have undergone, it is 

 inferred that the load of superincumbent rock at the time of the 

 principal earth-movements was not great. 



In order to ascertain the age of the beds, we must go to the main- 

 land, where there is evidence that rocks precisely similar to those in 

 Bardsey had been altered to their present condition and denuded 

 before Lower Arenig times, as many fragments of the schistose 

 slates and grits, in the condition of sericitic phyllite, quartz- 

 granulite, etc., occur in a Lower Arenig breccia on the promontory 

 about 2 miles north-east of Bardsey. 1 From a consideration of the 



1 C. A. Matley, ' The Arenig Eocks near Aberdaron ' Geol. Mag. dec. 4 ? 

 vol. ix (1902) p. 122. 



