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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. Surrey).
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
grit, about 1,500 feet in thickness, and yielding Tri
lobites 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.)
The Skiddaw Slates attain a thickness of about 7,000 feet, and are considered to represent the Arenig group.
They constitute the lowest beds of the Cambrian (or Cumbrian) series seen in Lake District.
The peculiar cone in cone'structure is often met with in these rocks.
Ores of Iron, Copper, Cobalt, and Lead occur in places. Slate-pencils have been largely manufactured near Shap.
The beds are developed near Egremont, Cockermouth, Crummock Lake, Black Coomb, &c., and they occur over a large area in the Isle of Man.
The Graptolites form the most remarkable fossils of the Skiddaw Slates: these include the genera Dichograpsu8, Tetragrapsus, and Phyllograpsus.
Some Trilobites occur, also Lingula brevis, Palæochorda major, and Annelide burrows.
TRILOBITE Schists. (Murchison, 1834.)
The name is taken from the town of Llandeilo in Caermarthenshire, where the rocks were first described by Murchison.
They consist of bluish-grey and black micaceous and calcareous flags, with black shales or slates at the base. Associated with them are many igneous rocks. The total thickness of the series is estimated at 3,300 to 4,000 feet, including sometimes 2,500 feet of lavas.
The Llandeilo Flags near Builth rise in a boss in the midst of the Silurian rocks, which lie unconformably upon them.
Cader Idris is formed of the Llandeilo beds with their included felspathic ashes, &c.
The beds are well developed in Shropshire, in the Shelve district, &c., and are probably represented in Anglesea.
The fossils include Asaphus tyrannus, Ogygia Buchii, Calymene, Leptæna, Lingula, Orthoceras.
Fig. 3.--Section of Cader Idris.
1. Slate and Shale. 2. Felspathic Trap. 3. Slates with Felspathic Ashes. 4. Amygdaloidal Greenstone. 5. Felspathic Ashes with two bands of Slate. 6. Greenstone.
1. Slate. 8. Felspathic Trap. 9. Felspathic and Calcareous Ashes, and Slates.
At St. David's, and throughout Pembrokeshire, these beds, according to Mr. Hicks, may be divided into three groups :
Upper Llandeilo—slates and flags with interbedded igneous rocks. 2,000 feet.
Middle Llandeilo-Calcareous beds and black slates. 1,000 feet.
Lower Llandeilo—Black slates, with interbedded felspathic ashes and tuffs. 500 to 7,000 feet.
BORROWDALE SERIES (Harkness and Nicholson), or GREEN SLATES AND PORPHYRIES (Sedgwick).
CHLORITIC SLATE AND PORPHYRY. In Cumberland and Westmoreland, overlying the Skiddaw Slates, comes a great thickness of igneous rocks, called the
1 This section is borrowed from Mackintosh's Scenery of England and Wales, p. 150. The scale is one inch to a mile, and the dotted lines represent Cyfrwy, or the Saddle, to the west of Llyn-y-Gader, and the steep cliff behind Llyn Cae or Cau. (Mackintosh.)
Green Slates and Porphyries, capped by the Coniston Limestone. The series consists of slates alternating with porphyries and felspathic ashes, and conformably overlying the Skiddaw Slates. Mr. Aveline states that, as a whole, the beds consist of lavas and consolidated or altered ashes and breccias, which have been ejected from volcanic vents, and vary in lithological character from thick-bedded coarse breccia to an ash so fine in texture that, when well cleaved, it yields good slates. Hence the name "Green Slates and Porphyries,'—the porphyries being the lavas or altered ashes, in which a porphyritic structure has been developed. The thickness is estimated at from 7,000 to 10,000 feet.
Dr. Nicholson states that the lower portion of the group is not known to contain any organic remains, and may possibly be of the age of the Upper Llandeilo. The upper portion, however, contains a band of fossiliferous shales, with characteristic Bala fossils; and the group, he considers, may be partly of Upper Llandeilo, and partly of Lower Bala age. Fossils have been obtained by Professor Harkness and Dr. Nicholson in the shales of Dufton, Swindale, and Pusgillsometimes called the Dufton Shales, &c., after these localities. The fossils include two species of Corals, stems of Crinoids, Brachiopods, Lamellibranchs, Holopea, Bellerophon, Orthoceras ; and Crustacea, such as Agnostus, Calymene Blumenbachii, Homalonotus bisulcatus, &c.
The lowest rock belonging to this system is a red, argillaceous, fissile stratum, characterized universally by its mottled colours, abundant along the eastern shores of Derwentwater, especially about Barrow and in St. John's Vale; in both localities resting upon the Skiddaw slate. It is distinctly stratified, dipping to the south-east, and is of considerable thickness. It passes upwards into the Green slate. The
It is not clear, so Professor Hughes tells me, that these fossiliferous shales belong to the Green Slates and Porphyries.