644 EEV. J. r. BLAKE ON THE 



variations, but sometimes becoming deep. In other areas some 

 great centres of eruption must have supplied the angular fragments 

 and the dust to all the districts. One centre could not have been 

 far from the western district, where the mud is unstratified ; while 

 the spot where the current-washed materials were deposited was 

 the present Northern District, then still further north. We can, 

 perhaps, scarcely look to the same source for the materials in the 

 Eastern District ; still less can we suppose the Lleyn to have been 

 dependent on Anglesey. Where, then, are the eruptive centres ? 

 In the places where we should naturally look for them we often 

 find masses of granite. Possibly, then, the granite occupies the 

 eruptive centre, and has, at a later date, intruded itself into the space 

 thus left and amongst the remnants that lay close to the source. 

 "Whether we can thus fix the sources or not, we must recognize that 

 in the end the volcanic forces became localized in minor and separate 

 spots, where they are indicated by the presence of agglomerates, 

 such as are seen near Pen-bryn-yr-Eglwys, on the west side of 

 Cemmaes, near Llangefni, at Dinas Llwyd, at Careg Gwladys, and in 

 Bardsey Island. Since we have granite in association with some 

 of these rocks at Pen-bryn-yr-Eglwys, we Taaj be justified in looking 

 upon other minor outbursts as of similar age, such as those south of 

 Traeth Dulas, east of Parys Mountain, and at Llyn TrefwU. It 

 may have been also at the same time that all the granite was intruded. 

 It was possibly at a later date that the felsites, south of Bangor, 

 w^ere poured out. Apparently during the continuance of these 

 minor outbursts, hot siliceous springs in some places, and calcareous 

 ones in others, burst up through the previously laid ashes and formed 

 the quartz-knobs and sporadic limestones. The crystalline material, 

 when formed in the sea, which, as ever, was most powerful in the 

 north, was soon broken up and rolled : the calcite produced the oolite 

 of Llanbadrig, and the quartz the conglomerates of Borth Wen, but 

 the outburst of ashes still continued. 



Yolcanos usually indicate land or, at least, the borders of the 

 land. After their formation we have no more deposits here, but at 

 a remoter distance deposits still continued, at Bray on the one hand, 

 and in the Longmynd or the other. The end of the period will 

 have to be sought out in these localities ; but it is probable that 

 where the Monian is thickest the succeeding Cambrian will be 

 thinnest or absent. 



The greater part of Anglesey remained dry land throughout the 

 Cambrian period, the deposits of the latter not reaching much beyond 

 its eastern border. Anglesey doubtless, indeed, from its later pro- 

 ducts, supplied the materials of the Cambrian slates and grits, of 

 which the lower ones still bear recognizable evidence of their source. 

 We need not suppose that the materials were derived from those of 

 newer rocks than any ncJw seen ; for where the grey gneiss is exposed, 

 a vast mass of material must have been removed, and it was certainly 

 removed in Cambrian times, since the basal Ordovician rests on the 

 grey gneiss. 



These Ordovician beds prove that the sea once more crept over 



