Articoli correlati a An intermediate text-book of geology
Sinossi
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1899 Excerpt: ...unseen, but its highest beds pass up conformably into the base of the succeeding Ordovician in the Arenig and Cader Idris ranges. Beds believed to represent these Cambrian rocks of Merionethshire occur in the Snowdon region--Caernarvon, Llanberis, Aber, &c.--with very rare fossils (Conocoryphe viola, Lingulella Davisii). Cambrian rocks also occur in the neighbourhood of St David's, South Wales, where the following sequence has been worked out by Dr Hicks:--Upper Cambrian. Tremadoc Beds--with Palcoasterina Ramsayensis, Ctenodonta, &c. Lingula Flags--with LinguleUa Davisii, Paraholina spinulosa. Middle. Menevian. Beds--Upper, with Orthis Hicksii and Stenotheca; Middle, with Paradoxides Davidis; and Lower, with Paradoxides Hicksii. Solva Group--Paradoxides aurora, P. Solvensis, P. Harknessi. Lower. Caerfai Group--with Lingulella ferruginea, Leperditia Cambrensis, and fragments of Olenellus (?). The total thickness of the Cambrian in this area is between 8000 and 9000 feet. The lowest bed is a conglomerate which lies with apparent conformity upon the volcanic rocks of the so-called Pebidian. In the Welsh areas already mentioned the Cambrian deposits are all of the "greywacke" type, and of great collective thickness. In the other parts of Britain, however, they present a somewhat different lithological facies, and are much reduced in thickness. England.--In Shropshire, fossiliferous Cambrian rocks occur in two districts--(a) the Wrekin-Caradoc district, and (b) the Pontesford-Linley district--which lie on opposite sides of the broad ridge of the Longmynd. In the Wrekin-Caradoc area the Cambrian consists of three members--a basement quartzite ( Wrekin Quartzite), a middle sandstone (Comley or Hollybush Sandstone), and an upper shaly series (Shin...
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