190 
THE SILURIAN VOLCANOES 
BOOK IV 
the Geological Survey tliey are thickest towards the west, and become 
more split up with intercalated sediments as they range eastward. 
Volcanic eruptions in this Shropshire region continued from the Arenig 
into the Bala period. They are marked among the Llaudeilo strata by 
occasional tufts and Ijy two massi\'e beds of “ volcanic grit,” described by 
Murchison,^ but they appear to have been rather less vigorous in the 
interval represented by tliis subdivision of the Silurian system. Those of 
Bala time gave forth abundant discharges of ash, of which tlie lowest 
accumulation, locally known as the Hagley Asli, con.sists of andesitic 
detritus. Occasional layers of tuff are intercalated in the overlying Hagley 
Shales, above which comes an important band called the Whittery Ash, 
“ consisting of andesitic and rhyolitic breccias and conglomerates, fine ashes 
with curious spherulitic or pisolitic structures, and bands of shale often 
tossiliferous.” ^ It is evident that the eruptions of the Shelve district came 
from independent vents in that neighbourhood, and never reached the 
importance of the great volcanoes of Arenig age in Montgomeryshire or of 
Bala age in Caernarvonshire. 
XumeroTis dykes and sills traverse the rocks of this district. They 
consist cliiefly of hypersthene-dol^rite. They appear to belong to a much 
Corndon, 
A, Arenig Hags and shales ; 13, ande.sites and tufla ; C, intrusive dolerite. 
later period than the interstratitied volcanic series ; at least some of them 
are found altering the I’entamerus limestones, and these must be later than 
tlie Llandovery rocks.^ The most important sill is that wliich forms 
Corndon, the central igneous mass of the district. This body of dolerite 
was ascertaiued by Mr. AVatts not to be a boss but a laccolite, which 
wedges out both towards the north -w’cst and south-east, as shown in 
Fig. 50. 
Six miles to the north of the Shelve and Corndon district the Breidden 
Hills rise on the border of Shropshire and Montgomeryshire, and include a 
mass of volcanic material belonging to a distinct area of eruption. In the 
ridge that extends for about three and a half miles through Moel-y-golfa 
and Middletown Hill, a synclinal trough of volcanic rocks lies upon shales, 
which from their fossils have been placed in the Bala group. The volcanic 
series appears to exceed 1000 feet in thickness. The lowest part of it on 
Moel-y-golfa consists of andesitic lavas about 400 feet thick, followed by^ 
tuffs and volcanic conglomerates. The lavas resemble some of the “ por- 
■ Silurian System, p. 229. - Messrs. Lapwortli and tVatts, op. cH. p. 318. 
After Prof. Lapwortli and Jlr. Watta, op. eit. p. 342. * (}p. dl. p. 339. 
