E. TATE ON THE LIAS ABOUT KADSTOCK. 501 



A departure from the usual character of this limestone is exhibited 

 by a quarry-section near Tyning, about \ a mile west from Timsbury. 

 Here the Middle Lias is represented by a whitish marly limestone, in 

 some parts fissile, and like some White Lias and Ostrea-liassica beds 

 in other portions; it is similar to chalk-marl. It has no conglomerate 

 below, which so largely prevails throughout the Radstock district, but 

 rests on the denuded surface of the Poraminiferal clay, which varies 

 from 3 to 24 inches in thickness. The section is as follows : — 



Middle Lias : ft. in. 



1. (a & b) White marly limestone, full of Belemnites, Ammonites Bu- 



vignieri, A. Jamesoni, Bel. clauatus, B. elongatus, Pitonillus 

 conicus, Trochus Thetis, Spiriferina rostrata, Waldheimia 



cornuta, &c 2 6 



Lower Lias : 



2. (/) Brown clay. 12 



3. (g) Spiriferina-baxik, hardly recognizable, much phosphatic > to 



matter. J 4 



4. (h) Grey limestone with a dense phosphatic layer at top. A. 



BucMandi 1 3 



5. Clay parting 3 



6. (?') Hard grey limestone. A. Johnstoni 



7. Similar limestones. 



The distribution of the Ammonites, as well as the presence of 

 certain other mollusks, proves that the whole thickness of the yellow 

 limestone of Radstock belongs to the A. -Jamesoni series, and that the 

 subordinate divisions, characterized by A. armatus, A. Jamesoni, and 

 A. ibex, are as clearly marked within the 13 feet as they are in the 

 150 feet which they occupy on the Yorkshire coast. 



Mr. C. Moore was so sagacious as to perceive that the deposition of 

 this rock was not synchronous with that of the so-called marlstone of 

 Gloucestershire ; he writes* : — " There appears to be little doubt the 

 marlstone of this district occupies a different horizon from that at 

 Ilminster, which is unquestionably the uppermost in the Middle-Lias 

 series, whilst probably the former is at the very base of the series, 

 and represents the passage beds of the Lower into the Middle Lias." 

 Though a very decided opinion is here expressed, yet he elsewhere 

 wavers ; and as some errors of determination have given a show of 

 discordance in respect to the persistency of Ammonite-zones, there 

 seemed to be grounds for a revision of the whole facts. 



The revised map of the district, issued by the Geological Survey in 

 1871, indicates Lower Lias only over the area occupied by the 

 A.- Jamesoni limestones ; and as it would have been so contrary to 

 custom to represent a " marlstone " as a Lower Lias, I wrote to 

 Mr. H. B. Woodward, who assisted in the survey, for an explanation, 

 which is as follows : — " The Middle Lias was omitted because the 

 small scale of the map did not allow of its being shown." Many 

 species from this rock are referred, in the Catalogue of Fossils, Geo- 

 logical Survey Museum, to the Middle Lias, whilst the equivalent 

 beds in Gloucestershire and Warwickshire,which are clays, are mapped 

 as Lower Lias. 



* M. and U. Lias, SW T . of England, p. 38. 



