« PreviousContinue »
on this somewhat tender subject of classification, I cannot do better than indicate those of both Sedgwick and Mur. cbison. At the same time, for the sake of convenience, I shall arrange the subject-matter according to that of the former geologist, the results of whose early researches amongst the older Palæozoic rocks of North Wales have been shown to be in all essential points correct, but have scarcely been adequately acknowledged.
It is perhaps needless to observe that the names Cambrian and Silurian are mere terms of convenience in classification : one name might indeed be applied to the whole series of rocks, but as we have no reason to doubt that the whole history of the earth was a series of regular changes, so the terms of classification, dividing the history into chapters and paragraphs, are useful divisions that assist the memory and mark out the leading modifications that have taken place over any given area. The divisions between Cambrian and Silurian strata mark no great change in the life history, and yet the classification now adopted is convenient as marking the greatest physical break yet noticed in the series in England and Wales.
The following Table shows the classification of the strata :
Bala and Lower ) Lower Silurian
(Sedgwick) | Arenig or Skiddaw
Group. Middle Tremadoc Beds
(Upper Cambrian Cambrian
Primordial (Sedgwick) Lingula Flags Ffestiniog Beds
Silurian (MurMaentwrog Beds
The total maximum thickness of the Cambrian strata is estimated at upwards of 30,000 feet in Wales, and at about 20,000 feet in the Lake district. But it must be remembered that owing to the disturbances and changes the beds have undergone, such estimates are very hypothetical.
The Cambrian rocks have, on the whole, been formed in shallow seas, but there are some evidences of tolerably deep water.
The Lower Cambrian rocks are often subdivided geographically, or according to the several tracts over which they are exposed. Two well-marked groups are, however, now established :
LONGMYND, OR HARLECH GROUP.
The rocks of this group consist of a series of grey, purple, and red flaggy sandstones, conglomerates and shaly beds, having a thickness estimated at 4,000 feet in South Wales, and supposed to be over 8,000 feet in North Wales. Remains of Sponges, Annelids, Pteropods, Polyzoa, Brachiopods, such as Lingulella and Obolella ; also of Entomostraca, and Trilobites of the genera Conocoryphe, Paradoxides, Microdiscus and Plutonia, have been determined in these strata, mainly through the researches of Mr. Hicks.
These indicate the marine origin of the strata in which they occur. Professor Ramsay has suggested that some of the red or purple beds which are unfossiliferous, may have been deposited in inland waters, or lacustrine areas subject to occasional influxes of the sea.
Longmynd Rocks. (Sedgwick.)
BOTTOM Rocks.' 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 bave 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, Paraloxirles, 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 igpeous
. 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 violent contortions.
Leicestershire.—The gritty, conglomeratic, and slaty