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fection 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 Tremanhire, 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.
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.
The uppermost part of the Malvern Shale is called the Dictyonema Shales by Dr. Holl on account of its being characterized by the presence of D. sociale. He states that this Polyzoon occurs in North Wales above the Lingula Flags. Mr. Hicks places the beds with the Dolgelly rocks.
UPPER CAMBRIAN (Sedgwick).
LOWER SILURIAN (Murchison and Geol. Survey).
ARENIG ROCKS. (Sedgwick.)
LOWER LLANDEILO. (Murchison.)
The Arenig mountains of Merionethshire give their name to this formation, which consists of shales, slates, and sandstones.
The quartzose rocks called Stiper Stones in Shropshire. belong to this series: they extend for ten miles, from Shrewsbury to near Bishop's Castle, and were by Murchison originally considered as the base of the Silurian System (1833-4). The thickness of the Arenig Rocks is about 1,000 feet.
The fossils comprise Obolella, Ogygia, Calymene, and many species of Graptolites: the species were considered by Salter to be distinct from those of the overlying Llandeilo Flags, but at the same time he regarded the Tremadoc Group as the natural termination of the Middle Cambrian, and the Arenig Group as the true base of the Upper Cambrian or Cambro-Silurian.
The Stiper Stones consist of a.thick band of siliceous sandstones, in parts veined, altered and fractured, and occasionally passing into crystalline quartz-rock. They constitute the natural base of the Llandeilo rocks of the Shelve and Corndon district. (Murchison.)
The Arenig Rocks are well developed at Shelve, in the Arenig Mountains, and at St. David's. At this last-named locality Mr. Hicks describes the series as consisting for the most part of black slates, attaining a thickness of nearly 4,000 feet, and affording evidence of being deposited in deep
Mr. Hicks has there divided the series into three groups:
Upper Arenig, consisting of fine black shales and slates about 1,500 feet in thickness, and yielding many species of Trilobites, Gasteropods, Brachiopods, and Lamellibranchs.
Middle Arenig, consisting of slates, flags, and bands of grit, about 1,500 feet in thickness, and yielding Trilobites and a few species of Graptolites.
Lower Arenig, consisting of fine black shales and slates attaining a thickness of about 1,000 feet, and resting conformably upon the Tremadoc group. Amongst the fossils are many species of Graptolites, Trilobites, &c.
The slates of Skiddaw are of a dark bluish or black colour, and contain flaggy beds with veins of quartz, also chiastolite slate.
The slate is generally of very uniform texture, soft, finegrained, and very fissile, and has been employed in the vicinity of Keswick and Hesket Newmarket for roofing houses; but for this use it is not very suitable, as it easily perishes in the atmosphere. In consequence of its want of durability, the mountains of this slate have smoother contours, more uniform slopes, and a more verdant surface than those of the following series (Green Slates and Porphyries). (Phillips.)