86 



UPPER LIAS. 



13. The Zone of Ammonites communis. 



Synomjms. — Alum sliale," Young and Bird, ' Geol. of York./ p. 133. " Upper Lias" 

 of English authors. " Posidonien-Scliiefer," Roraer, ' Oolit. Geb./ p. 5, 1836. " Lias- 

 Schiefer," von Buch, 'Jura Deutsch., Berl. Akadem/ 1837. " Posidonien-Schiefer," 

 Quenst., ' Elotzgebirge,' p. 538. " 9' Etage, Toarcien (pars infer.)," d'Orbigny, ' Cours. 

 element, de Paleontol./ p. 463. "Die Schichten der Posidonomya Bronni" Oppel, 

 ' Juraformation,' p. 197. " Communis-bed," Wright, ' Quart. Journ. Geol. Soc.,' vol. xiv, 

 p. 25. 



In Gloucestershire this zone consists of bluish clay containing occasional and irregular 

 bands of nodular argillaceous limestone resembling " cement stones." Li the escarpments 

 of the Cotteswold Hills it attains in some places a thickness of from 100 to 200 feet, and 

 is there interposed between the sands or rock-beds of the Inferior Oolite and Marl- 

 stone. The Upper Lias clay is generally concealed by debris from superior strata, and 

 its position is therefore most readily ascertained by surface indications, such as springs 

 and marshes. As this clay-bed forms a retentive stratum at the base of the superincum- 

 bent porous strata, the rain, which falls upon the table-land of the Cotteswold Hills, after 

 saturating the Oolitic rocks and subjacent sands, bursts forth as springs along their slopes 

 and escarpments, at the junction of these beds with the impervious clay. All the springs 

 in this district arising from the drainage of the Inferior Oolite have their origin in this 

 arrangement of the strata. 



On the summits of Bredon, Alderton, Gretton, and Churchdown Hills, all outhers of 

 the Cotteswolds, we find sections of the lower or basement beds of the Upper Lias ; these 

 consist in general of the following sub-divisions : 



1st. Brown marly clays of variable thickness, according to the extent of denudation of 

 the upper beds ; they contain many of the fossils of our list. 



2nd. A band of nodular argillaceous limestone from six to eight inches in thickness, 

 called the " Eish bed," this stratum has yielded many interesting remains. I obtained 

 from a nodule at Gretton, a large and nearly perfect specimen of PacJii/cormus latirostris, 

 Ag. ? and from nodules at Alderton, Dumbleton, and Gretton, Le2)tolejns concentricus, 

 Egert., Teirafjoiiolejns discus, Egert., have been extracted. Wings and elytra of insects 

 have likewise been found in nodules at Dumbleton and Gretton, of which the most 

 remarkable is a fine Neuroptcrous wing belonging to Lihellula Brodiei, Buck. 



3rd. Is a thick bed of bluish mottled clay, several feet in thickness, and more or less 

 laminated, at Alderton, where I have seen it once well exposed; it contained a great 

 many small Gasteropoda, among them were Centhiiim, Bostellaria, TrocJius, and Natica ; 

 of Conchifera, I found Area, Leda, and Posidonoimja ; of Echinodermata I observed 

 Acroscdeaia crinifera, Quenst., Pseudodiadema Moorei, Wr,, Ophioderma, n. sp., and 



