— M 3 o 2 — 
citées montrent qu’en certains points on faisait descendre les 
Tilestones, et par conséquent VOld Red, jusqu’à la base de VUpper 
Liidlow Rock. 
C’est cette erreur, non encore reconnue comme telle, qui fut la 
principale raison déterminante d’une nouvelle modification de la 
limite supérieure du système Silurien et de l’adoption de la limite 
communément admise aujourd’hui en Angleterre. Voici, en effet, 
ce que déclare à ce sujet Murcliison dans l’édition de i 854 de 
Siliiria (^) : « Being compelled in my researches to draw a line of 
démarcation between tlie upper part of the Ludlow formation and 
tlie bottom of tlie overlying Old Bed sandstone, I formerly included 
tlie tilestones in the latter ; particularly as in most parts of the 
région they décomposé into a red soil, and thus afford a clear 
physical line of démarcation between them and the inferior rocks, 
which facilitated the construction of the geological map. — Even 
then however, the fossils which were figured as characteristic of 
such tilestones exhibited little else, as I showed, than species 
common to the Ludlow rock itself. This zoological fact, and 
subséquent researches in other parts of England, above ail those 
of Professor Sedgwick in Westmoreland, where the Upper Ludlow 
strata are much develo^^ed, hâve, for eleveu years, led me to 
classify these tilestones with the Silurian rocks, of which they 
form the natural summit. For even in their range from Shropshire 
through Hereford and Radnorshire, into Brecon and Carmathen- 
shire, whether they are of red or yellow colours, they are charged 
with Orthoceras bullatum, Chonetes (Leptaena) lata, Spirifer 
elevatus, Orthis lunata, Rhynchonella nucula, Cucullella ovata, 
Bellerophon trilobatus, B. expansus, Trochus helicites, Holopella 
(Turritella) obsoleta, and the minute bivalve crustacean Beyrichia 
tuberculata. AU of these are the most common fossils of the Upper 
Ludlow rock ; although a few of them descend as low as the 
Caradoc sandstone. » 
Dans les éditions suivantes de Siliiria (^), la raison tirée des 
observations faites dans le Westmoreland a disparu de ce para¬ 
graphe, bien que, comme nous ne tarderons pas à le voir, ces 
observations aient été le point de départ de la réforme. D’autre 
(^) Siluria, Ed. 2, pp. iSS-iSg. 
(^) Siluria : Ed. 3 , p. 149 ; Ed. 4 et 5 , p. 184. 
