File:The Quarterly journal of the Geological Society of London (14053364906).jpg

ASSOCIATED ROCKS OF THE BEEIDDEN HILLS.
535
Fig. 1. — Section across Middletown Hill. (Length, \ mile.)
C S'. Cambrian Shales,
V A. Volcanic ash.
CS", Cambrian Shales.
Q,:
YC"

quarries.

Lower Volcanic Conglomerate.
Upper Volcanic Conglomerate.
C. Little crags. //. Fault.
kaolin and quartz. This ash is again seen to the S. of Moel-y-
Golfa, and in one or two isolated spots to the N.E. It passes up
into andesitic conglomerate, and that, in a quarry at the foot of
the hill, into 10' of volcanic grit without pebbles, followed by sand-
stone and shale with hard grit bands containing Pentamerus ; a
faidt obscures the exact relations of the two groups.
The lowest beds seen in Moel-y-Golfa are lavas of an andesitic
type, well exposed in the great crags at the S.W. end of the hill.
Similar lavas are seen to succeed one another on the S.E. side, one
of the lowest of them being amygdaloidal, to a total thickness perhaps
of 400 feet, until, at about 650 feet from the summit, they are fol-
lowed by ashy beds and these by conglomerates, both of which wrap
round the S.W. end of the hill ; these are visible in the crags and
quarries on and near the Welshpool road. The ash, when not of
the china-stone type, is obviously formed of the same materials as the
lavas of the hill, and the conglomerates, in which I have measured
fragments 2 feet long, are composed of lumps of the same rock.
The hill is very abrupt at its S.W. extremity, and the ashes and
conglomerates do not thin out as they do northwards, but are re-
placed on the plain by the shales of Trewern, like those found else-
where beneath the conglomerate, suggesting that the beds are cut
off by a fault; but there is no additional evidence of this unless we
consider the absence of the black grit of Belleisle Brook to be such.
At Oefn, near Buttington, there is an inlier, probably of the same
rocks, associated with an intrusion of diabase. The quarries show
shivery grey shales interbedded with grits often baked to quartzite
and very much contorted by the intrusion *.
Prom this description .it will be seen that the centre of volcanic
activity was at Moel-y-Golfa, where the lavas occur and near to
which the ash beds are thickest, while further north the area was
for the most part submerged, becoming occasionally shallow for the
formation of conglomerates and even at times upheaved to receive

• Silurian System, 292.