Siluria : a history of the oldest rocks in the British Isles and other countries with sketches of the origin and distribution of native gold, the general succession of geological formations, and changes of the earth's surface / by Sir Roderick Impey Murchison.

  • Murchison, Roderick Impey, Sir, 1792-1871.
Date:
1867
Catalogue details

Licence: Public Domain Mark

Credit: Siluria : a history of the oldest rocks in the British Isles and other countries with sketches of the origin and distribution of native gold, the general succession of geological formations, and changes of the earth's surface / by Sir Roderick Impey Murchison. Source: Wellcome Collection.

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    thickness of fossiliferous schists and sandstones with some calcareous courses, which clearly represent the original Caradoc Shelly Sandstones, to bo de- scribed in the next Chapter f. The Llandeilo flags and schists of Pembrokeshire and Carmarthenshire, charged with the same characteristic Trilobites and other fossils, emerge everywhere from beneath overlying deposits, often of a more arenaceous type, which are of the age of the Caradoc formation. In Carmarthenshire the Llandeilo Flags are associated, as elsewhere, with igneous rocks, both stratified and eruptive, though not on the same scale or of that clear and demonstrative character which is exhibited in the tract of Shelve in Shrop- shire, and in the district between Builth and Llandegly. By reference to the Map, indeed, it will be seen that it is just where such igneous rocks abound, as marked by bright red colour (*), whether in Shropshire or in Wales, that the Llandeilo formation (2^) rises prominently to the surface, through the surrounding deposits. Thus, in proceeding from the Llandeilo tract to the north-cast, we first Llanwutyd Wells. S.E. (From Sil. Syst. p. 33G. Sec also woodcut, Sil. Syst. p. 343.) N.W. Lower Silurian slaty rocks. View in a combo below the Baths of Llanwrtyd. (From a drawing by Mrs. Traherne.) meet with decisive eruptive masses of porphyry and other rocks at and near the Wells of Llanwrtyd in Brecknockshire, and their black schists and t To the north and west of the Llandeilo dis- trict, and all along the western frontier of the Builth country, the boundary-line of my first map of the Silurian Ec'gion, which separated the Silu- rian rocks from the so-called Cambrian of that day, W'as purely arbitrary. It is merely to be regarded as a demarcation between rocks on the east and south, with which I was acquainted, and avast sZafyregion on the west and north—which I had not examined, and with whose order and fossil contents I was unacquainted, though in common with Sedgwick and other geologists of the day I believed it to he of older date than the Lower Silurian. Hence I majqied it all as Cambrian. The first rectification of this errom>ous view was made in 1842, bv Professor Bamsaj^ who observed that, instead of being succei'ded by lower rocks to the north and west, the Llandeilo Flags folded over in those directions and passinl under superior strata charged with fossils which IMr. Salter recognized as well-known types of the Caradoc or Bala beds. Hence the whole of those districts of South Wales came to be coloured as Lower Silurian by the Geo- logical Surveyors. (See Map.)