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.[161] The ground has in recent years been more carefully worked over by Mr. G. H. Morton[162] and Professor Lapworth.[163] [Footnote 161: _Silurian System_ (1839), chap. xix.; _Siluria_, 4th edit. (1867), pp. 26, 49.] [Footnote 162: _Proc. Liverpool Geol. Soc._ x. (1854), p. 62.] [Footnote 163: _Geol. Mag._ (1887), p. 78.] At the top of the Arenig group of this district lies a zone of well-stratified andesitic tuff and breccia (Stapeley Ash), with frequent intercalations of shales, and occasionally fossiliferous.[164] 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 augite- 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 successive parallel outcrops of them are exposed. According to the mapping of the Geological Survey they are thickest towards the west, and become more split up with intercalated sediments as they range eastward. [Footnote 164: Prof. Lapworth and Mr. W. W. Watts, _Proc. Geol. Assoc._ xiii. (1894), pp. 317, 337.] Volcanic eruptions in this Shropshire region continued from the Arenig into the Bala period. They are marked among the Llandeilo strata by occasional tuffs and by two massive beds of "volcanic grit," described by Murchison,[165] but they appear to have been rather less vigorous in the interval represented by this subdivision of the Silurian system. Those of Bala time gave forth abundant discharges of ash, of which the lowest accumulation, locally known as the Hagley Ash, consists 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 fossiliferous."[166] 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. [Footnote 165: _Silurian System_, p. 229.] [Footnote 166: Messrs. Lapworth and Watts, _op. cit._ p. 318.] [Illustration: Fig. 50.--Section across the anticline of Corndon.[167] A, Arenig flags and shales; B, andesites and tuffs; C, intrusive dolerite.]