114 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



and character of the vents from which the lavas and ashes of 

 Merionethshire were discharged. In the course of the mapping of 

 the ground the Geological Survey recognized that, as the greatest 

 bulk of erupted material lies to the east and south-east of the 

 region, the chief centres of emission were to be looked for in that 

 quarter, and that possibly some of the intrusive masses which break 

 through the rocks west of the great escarpment may mark the site 

 of vents, such as Tyddyn-rhiw, Gelli-llwyd-fawr, Y-Poel-ddu, llhobell 

 Fawr, and certain bosses near Arenig *. The distribution of the 

 volcanic materials indicates that there were certainly more than one 

 active crater. While the southward thickening of the whole 

 volcanic group points to some specially vigorous volcano in that 

 quarter, the notable thinning away of the upper tuffs southward 

 and their great depth about Arenig suggest their having come from 

 some vent in this neighbourhood. On the other hand, the lower tuffs 

 are absent at Arenig, while on Aran Mawcldwy, only nine miles to 

 the south, they reach a depth of 3000 feet. Still farther to the south 

 these volcanic ejections become more and more divided by inter- 

 calated bands of ordinary sediment. One of the most important 

 volcanoes of the region evidently rose somewhere in the neighbour- 

 hood of what is now Aran Mawddwy. There seems reason to sur- 

 mise that the sites of the chief vents now lie to the east and south 

 of the great escarpment, buried under the thick sedimentary forma- 

 tions which cover all that region. If we are justified, on strati- 

 graphical and petrographical grounds, in connecting the volcanic 

 rocks of the Berwyn range with those of Merionethshire, we may 

 speculate on the existence of a group of submarine vents coming 

 into eruption at successive intervals from the close of the period of 

 the Lingula Flags up almost to that of the Bala rocks, and covering 

 with lavas and ashes a space of sea-bottom at least forty miles from 

 east to west by more than twenty miles from north to south, or 

 roughly an area of some 800 square miles. 



But besides the materials ejected to the surface, this ancient 

 volcanic region was marked by the intrusion of a vast amount of 

 igneous rock between and across the bedding-planes of the strata 

 deep underground. One of the most prominent features of the 

 Geological-Survey map is the great number of sills represented as 

 running with the general strike of the strata, especially between 

 the top of the Harlech grits and the base of the • volcanic 

 series. On the north side of the valle}- of the Mawddach, between 



* Mem. Geol. Survey, vol. iii, 2nd ed. p. 98 ; see also pp. 44, 54, 58, 71. 



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