302 LOWER SILURIAN ROCKS OF WEST SALOP AND MONTGOMERY. 



Breidden Hills are thrown off at the high angle of 70°, dipping to the south-east, under thin, flaggy, 

 argillaceous and slightly micaceous sandstone, containing Cardiolce, Orthocera, and Gi-aptolites ; 

 and on the opposite side of the mountain, at Roughleigh, the same beds dip to the north-west, 

 though at a much lower angle. The slighter inclination of these strata is explained by their being 

 so much further from the trappean axis of the Corndon. (See the last chapter.) In the central or 

 highest parts, as for example near the Harp, the beds are very little inclined. 



In Powis Castle Park, and in all the ridges between it and the junction of the Berriew and the Se- 

 vern dull grey shale prevails, occasionally enclosing spherical concretions, the whole being undistin- 

 guishable from the Lower Ludlow Rock and Wenlock Shale in many other localities. The concre- 

 tions consist of impure black argillaceous limestone, varying in diameter from four inches to a foot, 

 and break with a conchoidal fracture, in which respect, and in containing strings of white and black 

 calcareous spar, they resemble the septaria of the London clay or the nodular cement stones of the 

 Lias. The nucleus of these concretions consists sometimes of calcareous spar, quartz crystals, car- 

 bonate of copper, with occasionally minute nests and flakes of anthracite. Other concretions are 

 formed around organic remains, as the Asaphus caudutus, Calymene Blumenbachii, Orthocera, &c. ; 

 and when the natural divisions of these organic remains are occupied by any of the above simple 

 materials, they are beautifully diversified 1 . The bands of argillaceous sandstone which lie in the 

 troughs between the valleys of the Severn and Ffyrnwy (see section, PI. 32. f. 9.) present nothing 

 remarkable; and the hill of Yr-Alt may serve as a sample of the whole. Occasionally they pro- 

 duce a flagstone, which is durable if not exposed to the atmosphere. The quarries on the south- 

 eastern face of Golfa Hill, and those in the western sides of Powis Castle and in the bed of the 

 Berriew river near its junction with the Severn, are among the best examples, the stone being 

 usually of a dark indigo grey colour, sometimes slightly calcareous, and frequently marked by white 

 veins of carbonate of lime and quartz. The varied inclination of the strata composing the chief 

 masses of these " mudstones " in Montgomeryshire and the adjoining parts of Salop will be best 

 understood by reference to the map and sections, the dip being of course usually highest near to 

 those anticlinal lines along which the inferior strata are thrown up in numerous parallel ridges. 

 (PI. 32. f. 9.) 



Lower Silurian Rocks, or Caradoc Sandstone, and Llandeilo Flags of West Salop and 



Montgomery. 



The Lower Silurian Rocks to the west of the Longmynd are prominently exposed in 

 the upland district of Shelve and Corndon, where they have been described as alter- 

 nating with ridges of trap rock (Chapter 22.). Many of these ridges consist of the 

 Caradoc sandstone, others of flagstones similar to those at Llandeilo. The uppermost 

 members of these Lower Silurian Rocks in the form of impure sandy limestones, like 

 those described p. 217., containing the Pentamerus oblongus, Productus sericeus, and 

 other well-known fossils, fold round the highly-inclined edges of the older Cambrian 

 rocks of the Longmynd or oldest mineral axis of Shropshire. Examples are seen 



1 Similar concretions are characteristic of the Lower Ludlow Rock in the neighbourhood of Ludlow and 

 Aymestry, near Elton, and also on the flanks of Deerfold Chase. (See p. 205.) 



