562 



J. F. BLAKE ON THE UPPER 



horizon for them than for the Rag of Brucdale ; for the fauna of the 

 Mont des Boucards contains seven species usually found on higher 

 horizons, and that of Brucdale one only, the species of downward ten- 

 dency being equally divided. The surface of the Eag of Brucdale is 

 eroded ; and it was doubtless during the interval thus indicated that 

 the peculiar beds of the Mont des Boucards were deposited, while 

 the overlying marls spread over both areas. The fourth locality 

 is the valley of the Liane near Hesdin-1' Abbe and at Outreau, where 

 two borings were made, by which it is supposed to be proved that the 

 " limestones" of the Mont des Boucards underlie the Rag of Brucdale. 

 Beneath the latter, it seems, there were encountered some " limestones" 

 which were supposed to be identical with (51, 53) those of the Mont 

 des Boucards ; but when the proofs are examined, these are found to 

 consist solely in the fact that some shells were brought up which 

 were thought to be the Ceromya of the Mont des Boucards — a fact 

 not sufficient, even if the determination were right, to fix the horizon. 

 No other fossils appear to have been obtained, which is rather extra- 

 ordinary, considering the great abundance of Terebratula insignis at 

 the Mont des Boucards. The supposed proof is therefore utterly in- 

 adequate, and we may with more probability interpret the boring dif- 

 ferently. At the top (see 51, 53) we have, below the alluvium, 22*65 

 metres of undisputed beds, down to No. 18 • No. 18 is 6-40 metres 

 of the Nerinaean oolite of Hesdin-l'Abbe ; No. 19 and part of No. 20 

 (say 3 metres) are the grits with Trigonia Bronnii. The remainder of 

 No. 20 are the Ostrea-deltoidea marls, 12-90 metres ; Nos. 21-23, 

 which MM. Sauvage and Rigaux refer to a coralline limestone, though 

 there are no corals recorded, are attenuated representatives of the 

 Mont-des-Boucards limestones, not well developed and therefore 

 without the Terebratula insignis, 4*75 metres. No. 24 is the Coral 

 Bag of Brucdale with Cidaris florigemma, 6*60 metres. Nos. 25-29, 

 referred by MM. Sauvage and Rigaux to the Mont-des-Boucards lime- 

 stones on account of their containing a Ceromya at Hesdin-rAbbe, are 

 the marls above the Houllefort limestone, containing Cidaris flori- 

 gemma and many fragments of fossils ; and the poorly characterized 

 beds below are the ordinary Oxford Clay. 



The next bed presents no point for discussion ; it is a ferruginous 

 grit of little thickness whose characteristic fossils are Trigonia 

 Bronnii and Astarte communis (Morini). Its place, its fossils, and 

 its mineral character assimilate it at once to the sands of Glos, the 

 corresponding beds at Trouville, and the Sandsfoot grits. It is suc- 

 ceeded by the most constant limestone of this part of the Jurassic 

 series. Yaryingin thickness and somewhat in character, this latter 

 may nevertheless be easily traced from the extreme north near 

 Basinghen to the valley of the Liane. Like the others, it begins in 

 the north by being thin and somewhat sandy, with large-grained 

 oolites only here and there, and few fossils l>eyondTerehratida insignis. 

 It has gained considerable thickness near the Mont des Boucards ; 

 at Paincthun the oolite predominates, and at Hesdin-1' Abbe forms 

 thick masses which are extremely fossiliferous, the most noticeable 

 species being Nerinwa Desvoidyi, N. ccecilia, Pholadomya Jiortulana, 



