72 



LOWER LIAS. 



4, The Zone of Ammonites obtusus. 



Synonyms. — " Marston-Marble," Sowerby, 'Min. Con. Suppl. Index/ vol. i, 1812. 

 " Ammonite-bed (Lower Lias)," Murcbison, ' GeoL of Cheltenham,' 2nd edit., p. 42, 1845. 

 " Turneri-Thone," Quenstedt, 'Flozgeb. Wilrttembergs,' p. 540. " Sal)Ie d'Aubange 

 (pars infer.)," Dewalque et Chapuis, ' Luxembourg,' p. 12. " Gres de Virton (pars infer.)," 

 Dewalque, 'Lias de Luxembourg,' p. 48. "Die Schichten des Ammonites ohtiisus," 

 Oppel, ' Juraformation,' p. 50. "Indurated marl and limestone-beds," De la Beche, 

 " Section," &c., ' Geol. Trans.,' 2nd ser., vol. ii. Zone of Ammonites obtusus, Wright, 

 ' Quart. Journ. Geol. Soc.,' xvi, p. 404. 



Gloucestershire and W anvicksJiire. — The beds constituting this zone are well developed 

 in the Vale of Gloucester, and were exposed in the deep cuttings of the Bristol and 

 Birmingham Railway, especially near Bredon, from whence the best collection of the fossils 

 of these beds was obtained. The rocks consist of dark-gray or bluish shales and clays, 

 with irregular and inconstant beds of dark-gray argillaceous limestone, the shales being 

 in parts nodular or laminated, the clays thick and tenacious ; the nodular portions of the 

 shales were very fossiliferous. This zone forms part of the Cardinia-bed of the local 

 geologists of Warwickshire, where it appears to be exposed in several localities. Mr. Tomes' 

 collection contains some very fine specimens of Ammonites ohtusus, Am. multicostatus. Am. 

 Brookii, and Am. Sauceanus, d'Orb., obtained from the Am. obtusus beds; and Mr, 

 Kershaw's cabinet contains a series of Am. Sauzeanus, d'Orb., from Darlingstoke, near 

 Shipton-on-Stour. Mr. Etheridge has collected this species at Horfield, near Bristol. 



Dorsetshire. — At Lyme Regis the zone of Ammonites obtusus attains a considerable 

 thickness, and is well shown in the coast-section. The strata rise on the shore about half 

 a mile west of Charmouth, they consist of thick beds of dark marls, which rest upon the 

 Table-bed, formed by Broad Ledge. The lower part of these marls contains numerous 

 compressed Ammonites Birchii, Sow., and layers of nodules forming cement-stones. Above 

 these succeed shales and clays, thin bands of limestone, and thick beds of shale and marls 

 with mudstones. Above these again are inconstant bands of limestone containing septaria, 

 in which gigantic examples of Am. obtusus. Am. stellaris, and Am. Brooltii are found. 

 The following section shows the relative position of these beds. 



