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2. Coarse yellow sandstone, in places a conglomerate, and containing calcareous bands. (Hoar Edge grits.)
1. Sandy and argillaceous shales, sometimes fossiliferous.
The fauna of these beds is particularly rich in Trilobites and Brachiopods, amongst which may be mentioned Trinucleus concentricus, Orthis (Strophomena) grandis, Orthis vespertilio. Two genera of starfish, Protaster and Palæaster, are found at Bala.
In the Caradoc sandstone, remains of fossils are often so abundant as to render some of the beds sufficiently calcareous to be burnt for lime : these beds are known to the workmen as · Jacob's Stones. (Murchison.)
Some valuable deposits of phosphate of lime (phosphorite) have been discovered on the top of the Bala Limestone in North Wales. Beds of jasper occur at DinasMowddwy, and other places.'
Welsh oilstone is obtained from the vicinity of Llyn Idwal, Snowdon, and sometimes called Idwal stone. From Snowdon the • Cutler's green stone ’ is also obtained.
The Gore stone, which occurs near Kington in Radnorshire, is considered to be an altered rock of Caradoc age.
Hirnant Limestone. (Sedgwick.) This limestone is locally developed in the valley of Hirnant, south-east of Bala, and in a tortuous tract between Bala and Dinas-Mowddwy. It is described as a black pisolitic and fossiliferous limestone, containing species of Orthis (0. Hirnantensis, 0. sagittifera, &c.), Arca and Modiolopsis.
It occurs locally near the junction of the Bala Beds and the Upper Llandovery, and was considered of Upper Bala age by Sedgwick.
1 The Dinas sand is used for lining copper-furnaces.
Mevagissey.—The grey quartzites of Veryan Bay, first described by Murchison, have been considered by Sedgwick and M'Coy to be of Upper Bala age, the fossils from Carn Goran (Gorran Haven) being Orthis calligramma, O. alternata, Calymene brevicapitata, C. parvifrons, Homalonotus bisulcatus, &c. They were first made known by the collections of Mr. Peach. The rocks at Bolt Head, South Devon, are metamorphosed Devonian slates.
LOWER CONISTON GROUP.
CONISTON LIMESTONE. (Sedgwick.) This group consists in its lower division of bluish-grey limestone and calcareous slate, and above of flagstone and slate (Ash Gill or Lower Coniston Flags), generally calcareous, having a total average thickness of about 300 feet.
It is placed on the horizon of the Bala beds, but does not contain so great a development of limestone nor so many fossils as do those rocks. The beds rest conformably upon the Green Slates and Porphyries.
The Coniston Limestone stretches from the estuary of the Duddon to near Ambleside and Wastdale Crag. It occurs also in the Furness district, where the formation was called Ireleth limestone by Sedgwick; and it may be studied on the western side of Trout beck.
The Coniston Limestone is rich in fossil Corals, Brachiopods (Leptæna, Orthis, Strophomena), species of Orthoceras, and Crustacea (Agnostus, Calymene, Cheirurus, Illanus).
The upper portion of the series, consisting of slaty beds, is termed Trinucleus and Strophomena shales by Professor Hughes, from the occurrence in them of these fossils.
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 origi., 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 :
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 l'pper 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 40-) feet thick.
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.