iNNIVERSAIlY ADDRESS OF THE TRESIDENT. I I 5 



IJarmoiith and Illiaiadr ^Nrawddach, in a distance of twelve miles Uio 

 Survey mapped " more than 150 intrusions varyinf]^ from a few yards 

 to nearly a mile in length " *. This zone of sills is ecjually marked on 

 the south side of the valley. It may be traced all round the Harlech 

 anticline until it dies out, as the bedded masses also do, towards 

 Towyn on the south and round about Tremadoc on the north. 



The presence of such a zone of intrusive sheets at the base of an 

 ancient volcanic series has been noted in various parts of Britain and 

 in rocks of different geological ages up to the Tertiary basaltic 

 plateaux of Antrim and the Inner Hebrides. But nowhere, perhaps, 

 is it so strongly developed as beneath the Arenig group of lavas and 

 tuffs in North Wales. Abundant as are the protrusions marked on 

 the Geological-Survey map, they fall short of the actual number to be 

 met with on the ground. Indeed, to represent them as they really 

 are would require laborious surveying and the use of maps on a far 

 larger scale than one inch to a mile. 



The vast majority of these sills are basic rocks or, in the old and 

 convenient terminology, " greenstones." Those of the Cader-Idris 

 district have been examined by Messrs. Cole and Jennings, who find 

 that, notwithstanding the considerable alteration everywhere shown 

 by the abundant epidote and calcite, the coarser varieties may be 

 recognized as having originally been dolerites approaching gabbro, 

 with a well-developed ophitie structure, the general range of struc- 

 ture being from dolerites without olivine and aphanites to andesitio 

 rocks with an originally glassy matrix t. Dr. Hatch confirms this 

 diagnosis from the slides prepared from my specimens. The ophitie 

 structure is usually characteristic and well preserved, in spite of the 

 alteration indicated by epidote, chlorite, uralite, and leucoxene. 



That this zone of " greenstone " sills belongs to the period of the 

 Merionethshire volcanoes does not admit of serious doubt. The 

 way in which they follow the line of the great escarpment, their 

 almost entire absence from the Cambrian dome to the west, their 

 cessation as the overlying lavas and tuffs die out, and their scarcity 

 above the lower part of the volcanic group, show their close relation- 

 ship to that group. Moreover, that they must have been as a whole 

 later than the main part of the lavas and tutfs may be confidently 

 inferred from their position. The molten material of which they 

 were formed could not have forced its way between and across the 

 strata unless egress to the surface had been impeded by some thick 



* Mem. Geol. Survey, vol. iii. 2nd ed. p. 26. 



t Quart. Jouru. Geol. See. vol. xlv. (1889) p. 432. 



