THE 
GHOLOGICAL MAGAZINE. 
INEWARSERIES ST DECADE Tl” VOL Xe 
No. VIII.—AUGUST, 18953. 
(QystalSribiNni 7A lS APG nas. 
I.—Tue Rocks oF THE VOLCANO OF Ruosety Fawr. 
By Grenvitte A. J. Corz, M.R.I.A., F.G.S. 
Professor of Geology in the Royal College of Science for Ireland. 
HE conclusions as to the stratigraphical relations of the volcano 
of Rhobell Fawr, north of Dolgelley, in Wales, which were 
arrived at by Mr. T. H. Holland and myself,’ have received kindly 
recognition from Sir Archibald Geikie in his Presidential Address to 
the Geological Society of London in 1891;* and we may now con- 
sider that the great ring of Arenig volcanoes, Cader Idris, the Arans, 
Arenig Fawr, and Moelwyn, rose upon an area already shattered by 
sporadic Cambrian outbursts. After again visiting Rhobell Fawr 
in 1892, in company with Mr. Mervyn Marshall, I feel able to give 
some account of the rocks that build up that volcano and its outlying 
fort-like masses. 
The area, extending some four miles from north to south and two 
miles from east to west, is coloured generally as “greenstone” on 
the map of the Geological Survey, which was published in 1855; 
but it is probably well known that the mass has in reality a complex 
structure, including a few dykes and lavas, and immense accumu- 
lations of tuff and ash. Mr. Clifton Ward wrote,’ “The rocks of 
Rhobell-fawr, with the exception of a few small patches of undoubted 
greenstone, seem to be mainly of volcanic origin. The greatest 
thickness of beds exposed in aclearly continuous section, from the 
base on the south side of the summit, consists of alternations of 
coarse ash, breccia, and slaggy or scoriaceous breccia, with which 
are mingled in many parts numerous crystals of hornblende, some 
of them attaining a very large size.” Mr. Ward recognized, particu- 
larly on account of the prevalence of hornblende crystals, that the 
material erupted was “of a slightly different nature to that now 
forming the widely spread felstone and ash-beds of Arenig and 
Llandeilo age.” 
I. Tur Aveite-APHANItTEs. 
There are but scanty representatives of the deep-seated rocks of 
Rhobell, and most of these have cut through the later ashes. Yet 
they have a general affinity with the fragmental materials of the 
1 “Qn the Structure and Stratigraphical Relations of Rhobell Fawr,’ Gxzou. 
Mac. 1890, p. 447. 
2 Quart. Journ. Geol. Soc. vol. xlvii. Proceedings, p. 107. 
3 Mem. Geol. Survey of Great Britain, vol. iti. 2nd edit. p. 58. 
DECADE III.—VOL. X.—NQ. YIII. 22 
