Abstract
LONDON. Geological Society, November 9.—Mr. R. D. Oldham, president, in the chair.—L. D. Stamp and S. W. Wooldridge: The igneous and associated rocks of Llan-wrtyd (Brecon). Pt. 1: Stratigraphial (L. D. S.). The succession of rocks is given; the fossils from the lower horizon include Dicranografttus rectus, Hopkinson, Glyptograptus teretiusculus, var. siccatus, Elles and Wood, and Climacograptus schärenbergi, Lapworth; those from the higher horizon include Dicellograptus sextans, Hall, and var. exilis, Elles and Wood, and Glyptograptus teretiusculus, var. siccatus, Elles and Wood. Both assemblages are characteristic of the Dicranograptus shales of South Wales. The volcanic rocks of Llanwrtyd are therefore of lowest Bala (Survey classification) and on the same horizon as the upper basic and upper acid series of Cader Idris. The igneous rocks are cut off on the west by a fault, into which an intrusive mass appears to have been forced. Pt. 2: Petrographical (S. W. W.). The Lower Ashes are an acid series, of which the most characteristic member is a coarse flinty breccia. The spilites show pillow-structure in the upper part, but pass down into massive, finely vesicular rocks. The spilites are locally associated with spilite-breccias, consisting of angular fragments of various rocks and rounded bombs, of all sizes, of spilitic material. The bands of fine ash frequently interbedded with the sediments form dark flinty rocks weathering white. The intrusion is an enstatite-bearing rock of doubtful affinities.—L. D. Stamp: The base of the Devonian, with special reference to the Welsh borderland. The Ludlow Bone-bed forms a natural base: it consists of fish remains, all of which first appear at this horizon, and are genetically connected with higher Devonian faunas; it passes laterally into a conglomerate, and thus forms a natural physical base; it marks a palæontological and lithological break which can be correlated all over north-western Europe. The fauna of the lower beds (Ludlow Bone-bed, Downton-Castle Sandstone, and Platyschisma Shales) falls into three groups: (a) Upper Ludlovian marine species which survived the change of conditions indicated by the bone-bed, but gradually died out; (b) species which flourished for a short time under the changing conditions; and (c) new forms, chiefly fishes, which persist, or are closely connected with later Devonian forms. It is suggested, from the association of the early Downtonian fishes with marine invertebrates, that the former could live in either salt or brackish water, but gradually became specialised.
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Societies and Academies. Nature 108, 453–454 (1921). https://doi.org/10.1038/108453a0
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DOI: https://doi.org/10.1038/108453a0