The Quarterly journal of the Geological Society of London (1881) (14584051379)

Identifier: quarterlyjourna371881geol (find matches)

Title: The Quarterly journal of the Geological Society of London

Year: 1845 (1840s)

Authors: Geological Society of London

Subjects: Geology

Publisher: London (etc.)

Contributing Library: Smithsonian Libraries

Digitizing Sponsor: Biodiversity Heritage Library

Text Appearing Before Image:

stwyth grits; but nearly all theremaining rocks in the line of section might, I believe, have beenincluded in his second great series—the Plynlimmon group. Onthe other hand, I have considered this great Plynlimmon group ofSedgwick under two distinct headings, namely the (inferior) slateseries or Metalliferous-slate group, and the overlying Plynlimmongrits, so that we now have the following succession of deposits:— (3) The Plynlimmon grits, forming a line of high countryin the centre of Wales, including Plynlimmon.f (2) The Metalliferous-slate group, forming a broad zone ofCardiganshire J contorted country on each side of (3). group. 1 (1) The Aberystwyth grits, best developed between Aberyst-(_ wyth and Aberayron. Nos. (1) and (2), being closely bound together by their fossils, are knowntogether as the Cardiganshire group. Por an illustrative section of our area the best I can offer is thatfrom Aberystwyth, through the Devils Bridge, to the Plynlimmon GEOLOar OF CENTRAL WALES. 143

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144 WALTER KEEPING ON THE Mountains (figs. 1 & 2). The lowest beds form the cliffs at the coast,while the highest go to form the central mountains. All these rocksare amazingly contorted; and we find the intermediate or Metallifer-ous group especially thrown into frequent and violent twistings,with, in many places, actual inversion. A continuation of the sectioneastward, beyond Plynlimmon (fig. 2), shows a similar appearance ofthe rock groups in reversed order of outcrop and with contrary dips,thus showing that Plynlimmon, like its more ancient and greaterfellow Snowdon, stands in a geological valley or synclinal. Weproceed to the detail of this section. 1. The Aberystwyth grits consist of beds of hard, compact, darkgrey grit or greywacke, and dark shales, rabs, and imperfect slatesin strikingly regular alternation, as may be seen in the ordinaryphotographs of the Aberystwyth cliffs. The grey grits or greywackes are of great sameness and regularityboth in structure and composition

aberystwyth grits group geological cross sections of wales the quarterly journal of the geological society of london 1881 book illustrations natural history geology high resolution images from internet archive united kingdom

1881

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