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UPPER SILURIAN. (Murchison and Geol. Survey.)
THE term Silurian System, with which the name of Murchison will ever be connected, was given in 1835 from the country of the ancient Britons known as Silures; and it is, for the reason stated in the previous chapter, here confined to the rocks between the Caradoc or Bala beds and the Old Red Sandstone. The beds rest unconformably upon the Cambrian rocks.
The total thickness of the system may be as much as 6,000 feet, but it is by no means easy to give the thickness with accuracy: it varies from 3,000 to 6,000 feet, for the beds themselves are subject to much local change. Sedgwick has observed that where the Woolhope limestone is well developed, as at Presteign, there the Wenlock limestone is very feebly represented. At Wenlock the limestone forms a grand terrace, but the Aymestry limestone has almost vanished. At Leintwardine the Aymestry limestone is a grand rock, and the Wenlock limestone is but poorly developed.
The organic remains of the Silurian rocks indicate their marine origin, and show that the beds were deposited in a continuously but slowly subsiding area, and in water never of very great depth. Some of the limestones are the remains of old coral reefs.
The following Table exhibits the main divisions of the Silurian strata :
LOWER LLANDOVERY ROCKS.
These rocks, which consist of hard grey grits and conglomerates, with beds of slate and shale, were formerly included among the Caradoc Beds.
Professor Ramsay remarks that in mineral character these beds so strongly resemble the Upper Llandovery rocks, that up to 1856 no geologist had been able to distinguish between them. They attain a thickness of from 600 to 1,000 feet, or even 1,500 feet. The stratigraphical evidence, as Professor Hughes tells me, shows that Llandovery Rocks rest unconformably on beds older than any Llandovery Rocks, but it does not seem to be shown by reference to any localities that Upper Llandovery is unconformable to Lower Llandovery.
Casts of Pentamerus oblongus are common in this rock, near Wrexham and elsewhere.
Near Builth and Llandovery the beds appear to be most. fossiliferous. But, I am informed by Professor Hughes, that there is no trustworthy list showing what fossils are peculiar to Upper and what to Lower Llandovery; and while palæontologists cannot separate the fossils of the two formations into well marked groups, stratigraphical geologists confess the difficulty of separating them in the field. Therefore he would
regard them as forming one distinct group, the May Hill Group, and include with them the Tarannon Shale, in part.
The Lower Llandovery strata appear south-east of Bala Lake, and are developed over a great part of South Wales, near Rhayader, Garth, &c. The junction with the Upper Llandovery beds is seen near Noeth Grug, north-east of Llandovery. These rocks extend over a good deal of country on the borders of Cardigan Bay, between Aberystwith and Cardigan, and are exposed at Haverfordwest.
UPPER LLANDOVERY ROCKS OR MAY HILL SANDSTONE. PENTAMERUS BEDS. (P. oblongus zone.)
These rocks consist of grey and yellowish sandstones and conglomerate, attaining a thickness of about 800 feet. Sometimes a calcareous band is met with: as for instance the Norbury Limestone and Hollies Limestone.
The Upper Llandovery Beds of Murchison were at first considered to be the highest beds of the Caradoc Sandstone, and conformable with it: their true position was determined by Sedgwick.
In Shropshire, according to Messrs. Salter and Aveline, the 'Pentamerus beds' comprise :
Purple shales, 200 to 400 feet thick.
Thin limestone bands, interstratified with ochreous sandstone and argillaceous shales (Pentamerus beds proper).
Conglomerates and sandstones.
In North Wales the Upper Llandovery rocks are absent, and the Bala beds, as far north as the ground 6 miles southeast of Bala Lake, are overlaid by a long strip of grit probably belonging to Lower Llandovery strata, and this is overlaid by pale-grey, purple, and green Tarannon shales.
The May Hill Sandstone yields species of corals, Favosites, Heliolites, and Petraia; Annelides, Trilobites, and Mollusca, the Brachiopoda being most abundant. Amongst the species are Pentamerus lens, P. oblongus, Atrypa reticularis, Strophomena compressa, Bellerophon trilobatus, Encrinurus punctatus, and Petraia subduplicata.
In South Wales these beds first appear in Marloes Bay, and, at intervals, range across Pembrokeshire; but further north and east they disappear for a space, being overlapped by the Old Red Sandstone. They re-appear south of Llandeilo, and varying from a few feet to 1,000 feet in thickness, they range north-east in a narrow strip through parts of Caermarthenshire, Breconshire, and Radnorshire, lying indifferently upon Lower Llandovery, Caradoc, or Llandeilo Beds. Near Builth, only a few feet thick, they rest quite unconformably upon the Llandeilo Flags and their associated igneous rocks. They are also found near Presteign, where they are locally called 'Corton grit.' They occur at Nash Scar, and in Shropshire they lie very unconformably on the Caradoc Sandstone, between the neighbourhood of Cardington and Coalbrook Dale. In the Longmynd country they also lie quite unconformably in the form of a calcareous conglomerate on Cambrian rocks, and beyond this in Wales they are not known anywhere at the western base of the [Silurian] strata between Radnorshire and the mouth of the Conwy. (Ramsay.)
The Upper Llandovery rocks are developed in the Lower Lickey Hills in Worcestershire, and in Staffordshire; at the Lickey, low heathy hills occur chiefly composed of quartzrocks, lithologically identical with those masses on the flanks of the Caradoc and Wrekin which have been formed by the fusion of sandstone. (Murchison.)
They occur at May Hill, Shelve, and Tortworth. At Malvern the beds consist of grey and purple laminated sandstones and shales having a thickness of 500 feet, resting
upon grey and purple sandstones and conglomerates about 600 feet in thickness. (Phillips.)
TARANNON SHALE. (Geol. Survey.)
These beds comprise smooth pale-blue and greenish-grey slates and shales 1,000 to 1,500 feet in thickness.
They were first mapped by Mr. Aveline. These shales form the lowest part of the Silurian rocks of North Wales, and from beneath the Denbighshire grits they are exposed in a narrow and nearly unbroken line from the mouth of the Conwy to near Builth in Radnorshire, where they are strikingly unconformable to the various underlying members of the Cambrian strata.
In South Wales they rest conformably upon the Upper Llandovery rocks. No fossils have been found in the beds.
Graptolitic Mudstones. (Harkness and Nicholson.)
The Stockdale slates (which occur in the Lake District) are of a pale colour, passing downwards into black shales. with graptolites, and containing calcareous grit and conglomerate.
They are placed on the horizon of the Tarannon Shale. Amongst the fossils are Graptolites, Diplograpsus, Rastrites, &c., also Endoceras.
Dr. Nicholson states that the beds rest in the Lake District upon the Coniston Limestone, and in the Sedbergh District upon the Trinucleus and Strophomena Shales.