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.[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.]

A, Arenig flags and shales; B, andesites and tuffs; C, intrusive dolerite.]

[Footnote 167: After Prof. Lapworth and Mr. Watts, _op. cit._ p. 342.]

Numerous dykes and sills traverse the rocks of this district. They consist chiefly of hypersthene-dolerite. They appear to belong to a much later period than the interstratified volcanic series; at least some of them are found altering the Pentamerus limestones, and these must be later than the Llandovery rocks.[168] The most important sill is that which forms Corndon, the central igneous mass of the district. This body of dolerite was ascertained by Mr. Watts not to be a boss but a laccolite, which wedges out both towards the north-west and south-east, as shown in Fig. 50.

[Footnote 168: _Op. cit._ p. 339.]

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 "porphyrites" of the Old Red Sandstone, and contain two forms of pyroxene--one rhombic, probably enstatite, and the other monoclinic augite. There are likewise considerable masses of intrusive rock, which are varieties of diabase or dolerite.[169]

[Footnote 169: See Mr. W. W. Watts on the Igneous and Associated Rocks of the Breidden Hills, _Quart. Journ. Geol. Soc._ vol. xli. (1885), p. 532.]