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Longmynd Rocks. (Sedgwick.)
GWASTADEN (BRECON) ROCKS. (Murchison, 1834.)
These rocks, so named from the Longmynd hills in Shropshire, consist of green and purple grits, conglomerates and slates. They are developed at the Longmynd and in the neighbourhood of Shrewsbury, where they are overlaid conformably by the Lingula flags. Their thickness has been estimated at 8,000 feet.
The beds contain Annelid burrows, belonging to two species, according to Mr. Salter, who named them Arenicolites didymus and A. sparsus.
Harlech Grits. (Sedgwick.)
A large tract of ground between Barmouth and Harlech, extending eastwards to Craig-y-Penmaen in Merionethshire, is composed of greenish grits, interstratified here and there with green and purple slates. The series is stated by Professor Ramsay to be more than 6,000 feet in thickness. The beds are pierced by dykes of igneous rock.
Evidence of sun cracks and rain-drops has been detected on the surfaces of some of the beds.
Llanberis Grits and Slates. (Sedgwick.)
This series includes the famous slates of Penrhyn and Llanberis it comprises the altered purple and green slaty, arenaceous, and conglomeratic beds west and southwest of Bangor; together with the purple and green slates, and grits on the banks of the Ogwen around Bethesda, the lakes of Llanberis, &c. It attains a thickness of about 3,000 feet. (Ramsay.)
The beds contain some igneous dykes, while the cleavage is very intense and distinct from the bedding.
The large slate quarry of Penrhyn is probably familiar to all visitors to North Wales. The summit of the quarry is about 500 feet above the base, and the slates are worked out in terraces, each about 40 feet in height.
The beds have yielded no fossils save some burrows of marine worms termed Chondrites.
The green banding of the slates, and the production of the large uniform masses of green, are considered by Mr. Maw to be not only due to independent causes, but probably to have occurred at different times. The ordinary form of variegation of the slates consists of mechanically formed nuclei
concentrically environed by pale green slate, the bleaching of which has been due to the abstraction of the greater part of the colouring oxides of iron. Mr. Maw has pointed out that this banding and blotching of the slate was formed before the slate was cleaved, and some of the green slate was converted from the purple at the time of the intrusion of the greenstone dykes.
Caernarvonshire.-The Lleyn district (the promontory that separates Caernarvon Bay from Cardigan Bay) contains rocks believed to be of Lower Cambrian age. These are the metamorphosed rocks or schists on the south side of Caernarvon Bay, including Bardsey Island and the coast from Bardsey Sound to Porth Nevin. The age of these rocks has, however, been much questioned.
St. David's.-The grey and purple slates, grits and conglomerates associated with many igneous rocks which are developed on the north side of St. Bride's Bay in Pembrokeshire are of Lower Cambrian age, and comprise a series more than 4,000 feet in thickness. Mr. Hicks has discovered a rich fauna in these rocks, comprising many species of Trilobites, Brachiopods, &c., including species of Plutonia, Conocoryphe, Paradoxides, and Lingulella. The cathedral of St. David's is in some parts built of Cambrian sandstone.
Anglesea. The micaceous and chloritic schists, the gneissic rocks, grits and quartz-rock which constitute the greater portion of Anglesea, are believed to be of Lower Cambrian age, if not older. They are traversed by igneous dykes, and associated with bosses and veins of granite. The exact age of some of the strata on parts of the north coast of the island is by no means certain.
Holyhead Island and the North and South Stack islets are composed of Cambrian grits and schists, showing very
Leicestershire.-The gritty, conglomeratic, and slaty
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 Paradoxides 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.
PRIMORDIAL ZONE. (Barrande.)
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 Lingule.
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 estuary 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.
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 per