CHAP. XII 
ARENIC ERUPTIONS— MERIONETHSHIRE 
179 
as they are traced southwards from Cader Idris and north-westwards from 
Treinadoc.^ The greatest diameter of ground across which they are now con- 
tinuonsly traceable is about tw'enty-eight miles. They attain their greatest 
thickness, upwards of 5000 feet, in Aran Mawddwy, which rises from 
their most easterly escarpment. We may therefore infer that the main 
vent or vents lay somewhere in that direction. The noble range of preci- 
pices facipg westwards shows how greatly the limits of the volcanic rocks 
have been reduced by denudation. There can be little doubt that at least 
the finer tuffs extended westwards as far as a line drawn from Tremadoc 
to Llanegrin — that is, some fifteen miles or more beyond tlie cliffs of Aran 
Mawddwy, thus stretching across much of the site of what is now the 
great Harlech anticline. 
This compact, well-defined volcanic area, in spite of the faults which 
traverse it and the disturbed positions into which its rocks have been 
thrown, is, in many respects, one of the simplest and most easily studied 
among the Palaeozoic formations of this country. Its main features liave 
been delineated on the maps of the Geological Suiuey and have been 
described in Sir Andrew liamsay’s monograph. But these publications cannot 
be regarded as more than a first broad, though masterly, outline of the 
whole subject. There is an ample field for further and more minute 
research wherein, with the larger and better Ordnance maps now availalile, 
and with the advantage of the numerous modern petrographical aids, a 
more exhaustive account may be given of the district. The whole volcanic 
succession from base to summit is laid bare in innumerable magnificent 
natural sections along ranges of hills for a distance of some forty miles, and 
a careful study and re-mapping of it could not fail to add greatly to our 
knowledge of the early history of volcanic action.^ 
According to the observations of the Geological Survey, the Arenig 
Volcanic rocks of Merionethshire naturally arrange themselves in three 
great bands, each of which is described as tolerably persistent throughout 
the whole district; — 1st, a lower series of ashes and conglomerates, some- 
times ;5d00 feet thick (Aran Mawddwy) ; 2nd, a middle group of “fel- 
stones ” and “ porphyries,” consisting partly of true contemporaneous lava- 
streams and partly of intrusive sheets, and reaching a thickness of 1.500 
feet; 3rd, an upper series of fragmental deposits like that beneath, the 
extreme thickness of which is 800 feet (Arenig mountain). A re-mapping 
of the ground on the six-inch maps would, no doubt, show many local 
departures from this general scheme. 
The pyroclastic members of this volcanic series present many features 
ef interest both to the field-geologist and the petrographer ; luit they have 
'i-s yet been only partially studied. At the soutliern end of the district 
it is remarkaljle to what a large extent the earliest eruptions must have 
^ Op. ait. p. 96. 
The excellent papers of Profe.ssor Cole, Mr. Jennings, Mr. Holland, Mr. G. J. Williams, Mr. 
G Lake and Mr. S. H. Reynolds are illustrations of how the published work of the Geological 
Survey may he modified and elaliorated. 
