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rocks associated with syenite, and some other igneous rocks, that are exposed in the district of Charnwood Forest, may be of Cambrian age. Many of the rocks are highly metamorphosed, and so gradual has the change been, that the igneous and stratified rocks pass one into the other. (See p. 24.)
The term Menevian, from the old Roman name of St. David's, was proposed by Mr. J. W. Salter and Mr. Hicks for a series of black and grey slates and flags with thick beds of sandstone which underlie the true Lingula Flags, and attain a thickness of 500 or 600 feet. They contain many species of trilobites, amongst which the large Paralo.cides Davidis, sometimes nearly 2 feet in length, is conspicuous. Most of the characteristic fossils are as yet unknown in the beds above. One Cystidean and some Entomostraca make their appearance.
The Menevian beds are developed at St. David's in South Wales, and in the neighbourhood of Maentwrog and Dolgelly in North Wales.
At St. David's the beds are very fossiliferous, and so closely related palæontologically to the Longmynd group that Mr. Hicks (in 1867) proposed to class the two groups together as Lower Cambrian.
These beds were first named in consequence of Mr. Davis's discovery in 1845 of Lingula (Lingulella) Davisii in these rocks near Tremadoc. They consist of slaty and shaly beds
with grits and hard sandstones, often much altered. Where well developed they attain a thickness of from 5,000 to 6,000 feet; but half this estimate is sometimes considered sufficient. They pass insensibly into the Lower Cambrian rocks.
A pod-shrimp’ (Hymenocaris) and many Trilobites make their appearance in them.
The quartz-rock ridge of the Stiper Stones is underlaid by black and dark-blue slaty beds with Lingulae.
Many dykes and intrusive bosses of igneous rock penetrate the beds: such may be seen in the Ffestiniog Slate Quarries.
The Lingula beds are well developed in Merionethshire, ranging from the mouth of the Barmouth estnary to the north-east, and then circling round the Cambrian grits by Ffestiniog they pass out to sea on the south side of Traeth Bach.
They lie on the west side of the Longmynd; and may be studied at St. David's in South Wales, where they rest conformably upon the Menevian beds, and attain a thickness estimated by Mr. Hicks at 2,000 feet.
The gold lode of Dol-frwynog occurs in a talcose schist associated with igneous rocks on the horizon of the Lingula Flags.
In the maps of the Geological Survey, all the strata (excepting the Igneous rocks) from the Lingula Flags to the Lower Llandovery strata have received one colour; for though an order of succession can be made out by help of fossils, yet practically most of the formations pass so gradually into each other that it is impossible to define their limits on the map. (Ramsay.)
Maentwrog or Lower Lingula Flags.
OLENUS Beds. This name was proposed by Mr. T. Belt in 1867 for the slates and flags with bands of sandstone, characterized by typical forms of Olenus, and which are exhibited in great perfection at and around the village of Maentwrog. The Maentwrog group is specially characterized by its dark blue jointed ferruginous slates. It attains a thickness of about 2,500 feet. Agnostus nodosus and A. pisiformis occur with two or three species of Olenus. Phyllopoda make their appearance. The slates are sometimes worked for economic purposes. The goldmines of Hafod-y-morfa, Cefn-deuddwr, have been opened in these beds. Ores of copper, lead, and zinc occur in small quantities.
The Maentwrog beds extend from a little above Barmouth to Llanelltyd. They are well-developed in the Waterfall valley and in the valley running from Tafarn-helig to Caeny-coed.
They are seen at St. David's in South Wales.
Ffestiniog or Middle Lingula Flags. Mr. Belt proposed (1867) to restrict the name of Ffestiniog group to the hard sandy and micaceous flags containing Lingulella Davisii, and Hymenocaris vermicauda, which lie conformably upon the Maentwrog beds. This group is about 2,000 feet in thickness. Good building-stones and flags are worked in it; and copper-ore has been obtained at Glasdir.
The river Mawddach cuts through the whole of the beds between Rhiwfelyn and Hafod-fraith. They cross the Wnion near Glyn Maldon, and then by Gwern-y-barcud and Tyny.craig, range to Coed-y-garth, and into the estuary of the Mawddach. They occur at Ramsey Island, near St. David's.
Dolgelly or Upper Lingula Flags. The term Dolgelly group was proposed by Mr. Belt (1867) for the soft blue and black slates of the neighbourhood of Dolgelly. They are characterized by a small species of Orthis and by Parabolina (Olenus) spinulosa,
Mr. Belt states that the slates in the Upper Dolgelly beds give a black streak, and that with the exception of a thin layer of black slate in the Ffestiniog group, there are no other beds in the Dolgelly district so characterised.
The fossils in the Dolgelly group include many Trilobites, such as Conocoryphe, Sphærophthalmus, Agnostus, &c. A few molluscs also occur.
The beds are developed at Dolgelly and near Portmadoc. In South Wales the series comprises bluish and grey flags. Their thickness is estimated at 600 feet.
This formation, so called by Sedgwick in 1846 from the town of Tremadoc in Caernarvonshire, consists of blue and grey slates, flags, and sandstones, having a thickness of about 2,000 feet. They rest conformably upon the Lingula Flags.
Their occurrence was unknown out of Merionethshire until, in 1866, Messrs. Salter and Hicks indicated their presence near St. David's.
Many Trilobites are found belonging to the genera Ogygia, Asaphus, Conocoryphe, Olenus, &c. In this formation Cephalopoda first make their appearance, such as Orthoceras and Cyrtoceras; Hydrozoa, Crinoids, Asteroids, and Lamellibranchs appear. Lingulella Davisii is met with, also Conularia.
Mr. Hicks has made three divisions in the Tremadoc slates -the lower beds, consisting of grey flaggy sandstones, about 1,000 feet in thickness, occur at Tremanbire, Ramsey Island, and Llanveran, near St. David's. The middle division consists of dark earthy slates, and the upper division of ironstained slates and flags: these have a united thickness of about 1,000 feet, and occur at Portmadoc and Dolgelly.
The upper division he has in a table of Strata, 1873) placed in the lower part of the Arenig group.
Certain light green micaceous shales occurring at Cressage, near the Wrekin, have been identified with the Tremadoc rocks,
Rocks considered to be on the horizon of the Tremadoc Beds or Upper Lingula Flags occur at Malvern.
Hollybush Sandstone. This consists of greenish grey or brownish sandstone, with quartz-conglomerate at base, resting unconformably upon the gneiss of Malvern, and attaining a thickness of from 200 to 600 feet (see fig. 1, p. 24). Dr. Holl has found in it Annelids (Trachyderma antiquissima), and some Brachiopoda of the genera Lingula and Obolella.
The Hollybush Sandstone may be studied on Raggedstone Hill, overlooking the Hollybush valley, Malvern. Mr. Hicks places it on the horizon of the Ffestiniog beds.
Above the Hollybush Sandstone comes the Black Shale' of Malvern. This deposit consists of thinly laminated black carbonaceous and pale greenish shale, from 500 to 1,000 feet in thickness, enclosing some bands of trap, composed of felspar and hornblende.
The eruptive rock is an ancient lava, consolidated for the most part underground, or under the sea. The great deposit of shale must have been formed in calmer and probably deeper water than the Hollybush Sandstone, there having been doubtless a continual subsidence of the sea-bed interrupted by occasional volcanic outbursts. (J. Phillips.)
The Black Shale has yielded among the Trilobites, species of Conocoryphe, Olenus, Sphærophthalmus, agnostus; and among the Brachiopods, Lingula and Obolella.