CARADOC SANDSTONE AND FOSSILS OF WELCH POOL. 



303 



near Linley Hall, extending thence by Norbury to Wentnor, and at Cox's Mill. This 

 unconformable arrangement is of great interest in teaching us, that where those strata 

 which in other places form the beds of passage between the Silurian and Cambrian 

 systems are wanting, the contiguous rocks belonging to the two systems are uncon- 

 formable ; and it also seems to prove, that such Cambrian rocks had been placed in 

 inclined positions, before the deposition of the Caradoc sandstone had commenced. 



At the north-western edges of the Long Mountain, between the Black Moor and the Ffron Gate, 

 are red and green sandy shales, containing the same species of Lingula as the shale at Buildwas. 

 These red and green beds so contiguous to the coal-field of Shrewsbury might well be mistaken for 

 the Old Red Sandstone; but the included organic remains and the dip of the strata beneath beds 

 inclosing fossils of the Wenlock shale completely decide the case. On the south-western flanks of 

 the Long Mountain is not only the usual thick development of Upper Silurian Rocks, namely, a 

 flag-like mudstone with a lower shale containing concretions of argillaceous limestone, but also 

 thick-bedded, hard, and greyish blue grits, wrought into square blocks of good and durable quality. 

 These, on exposure, frequently become of a rusty brown colour. Judging from their mineral cha- 

 racter, I am disposed to place these grits in the Caradoc formation. 



Beds of calcareous sandstone, with fossils similar to those on the eastern flanks of the Caradoc, 

 occur at the Cefn near the flank of the Breidden Hills, where they are penetrated by trap rocks. 

 (See last chapter, p. 293.) 



Another zone of these deposits is thrown up at Welch Pool through the Upper Silurian 

 Rocks, and extends from about a mile N.N.E. of that town to upwards of a mile S.S.W. 

 of it. These are upon the line of a trappean eruption before described (the Breidden, 

 Standard, &c). The Caradoc sandstones in this short range are of red and dark grey 

 colour, the former predominating. In the high ground north of Welch Pool, called the 

 Red Hill, are quarries in highly inclined and fractured strata of these rocks, some of 

 them consisting of hard grits, in parts calcareous, others of mottled, red and grey 

 sandy shale, the former predominating. In a woody dingle 1 between these old quarries 

 and the Standard or great trap dyke, the shale abounds with casts of beautifully orna- 

 mented trilobites of the genus Trinucleus, so abundant in the Caradoc sandstone of 

 other tracts. Some of the more calcareous beds are seen on the flanks of the Standard 

 trap dyke, and they again protrude in various detached masses in the park of Powis 

 Castle. One of these, near the north end of the park, is a grey, impure, sandy 

 limestone, in parts concretionary, the weathered surfaces of the concretions being 

 marked by shells, corals, &c. But the most, remarkable of these knolls is that on 

 Which Powis Castle is built, the edifice having been constructed from quarries in 



1 As this ravine had not, as far as I could ascertain, any name, I venture to hope that, to mark so interest- 

 ing a fossil locality, Lord Clive will call it " Trilobite Dingle." The large specimen, Asaphus Pourisii, named 

 in honour of the noble family, so justly beloved throughout Montgomeryshire, was found at this spot with 

 the Orthis testudinaria and other shells, and two or three species of the genus Trinucleus, including T. Carac- 

 tacus. (See description of these trilobites in the chapters on Organic Remains.) 



2 p 2 



