CHAP. XII 
ARE NIG ERUPTIONS— SHROPSHIRE 
189 
affects the Llaudeilo as well as older formations. This granitic boss has 
thus probably no connection with the Arenig volcanoes, but belongs to a 
later period in the volcanic history of the Principality. 
The remarkable scarcity of dykes in the volcanic districts of Wales has 
been noticed by more than one observer. Among the intrusive “ gxeen- 
stones ” of Merionethshire some occasionally assume the . dyke form, and 
through the agglomerates and tuffs of llhobell Pawr dykes ot olivine-diabase 
have worked their way. In the Festiniog district various altered andesitic 
dykes have been noted. But there has been no widespread fissuring of the 
ground and uprise of lava in the rents, such as may be seen in the Archaean 
gneiss, and in the later Palaeozoic, but still more in the Tertiary volcanic 
regions. This feature becomes aril the more notable when it is viewed in 
connection with the great development of sills, and the evidence thereby 
afforded of widespread and extremely vigorous sid.iterranean volcanic action. 
In the Merionethshire region there certainly was a long period of 
quiescence between the close of the Arenig and the beginning of the Bala 
eruptions. Moreover, no evidence has yet been found that active vents 
ever again appeared in that district, the subterranean energy at its next 
outburst having broken out farther to the east and north. In Anglesey, 
however, where, as I shall point out, there is proof of contemporaneous 
tuffs among the Arenig rocks, it is possible that a continuous record of 
volcanic action may yet be traced from Arenig well onward into Bala time. 
ii. SHROPSHIRE 
About 35 miles to the south-east of the great volcanic range of 
Merionethshire a small tract of Arenig rocks rises from amidst younger 
formations, and forms the picturesque country between Church Stoke and 
Pontesbury. Murchison in his excellent account of this district clearly 
recognized the presence of both intrusive and interstratified igneous rocks.^ 
The ground has in recent years been more carefully worked over by Mr. 
G. H. Morton and Professor Lapworth.® 
At the top of the Arenig group of this district lies a zone of well- 
stratified andesitic tuff and breccia (Stapeley Ash), with frequent intercala- 
tions of shales, and occasionally fossiliferous.^ There is thus satisfactory 
proof of contemporaneous eruptions at intervals during the accumulation of 
the later Arenig sediments. That there were also outflows of lava is 
shown by the presence of sheets of augito- and hypersthene- andesite. 
These volcanic intercalations form marked ridges, having a general northerly 
trend. They are folded over the broad laccolitic ridge of Corndon, on the 
east side of which they are thrown into a synclinal trough, so that succes- 
sive parallel outcrops of them are exposed. According to the mapping of 
1 Silurian System (1839), chap, xix.; Siluria, 4tli edit. (1867), pp. 26, 49. 
Proc, Liverpool Ocol, Soc. x. (1854), p. 62. ^ Gcol, May. (1887), p- 78. 
■* Prof. Lapwortli and Hr. W. W. AVatts, Proc. Geol. Assoc, xiii. (1894), pp. 317, 337. 
