564 



J. P. BLAKE ON THE UPPER 



one division, or zone, of Amm. longispinus. The upper portion is 

 very shaly, and contains, on both sides of Boulogne, Lingula ovalis, 

 Trigonia variegata, and Thracia depressa. The upper limit of this 

 is assumed by M. Pellat to be the remarkable lumachelle of Exogyra 

 virgula, which stands out as a broad band in the cliff of Chatillon, 

 but which unfortunately is absent from La Creche. The base of 

 the " Portland " was originally taken by M. Pellat (39), as by all 

 others, below the great conglomerates ; he subsequently included all 

 M as " Portlandian" (62), but finally (68) took only the upper half. 

 Certainly the beds become sandy by degrees, and, for some distance 

 below the conglomerates, begin to indicate coming changes, as may 

 be seen both at La Creche and Gris Nez. This change, being 

 accompanied by the introduction of a new fauna, including Ammo- 

 nites gig as, Carclium morinicum, Trigonia Munieri and T. barrensis, 

 Perna Bouchardi, and Hemicidaris purbecJcensis, may well justify 

 the lowering of the line of separation, as we so often have to do, 

 the palaeontological change being accomplished more quickly than 

 the lithological. 



The next portion of the series is perhaps the most interesting of 

 all ; and as it is chiefly exposed along the coast, we here reap the 

 benefit of M. Hebert's admirable and accurate researches, which 

 may be referred to for the closest details (37). He describes the 

 three chief localities where these rocks may be seen, viz. : — to the 

 south of Boulogne, from Chatillon to Equihen ; to the north, from 

 La Creche to Wimereux, and at Gris Nez. The shales last noticed 

 are the No. 26 of his sections, and from their thickness form an 

 admirable base. The lowest bed of the "Lower Portlandian" at 

 La Creche is a 3 to 4-foot band of excessively hard crystalline calc- 

 grit, blue in the interior and without fossils ; it is followed by grey 

 fucoidal doggers in sands or clays (not well seen) for 10 feet or 

 more ; above come 4 feet of hard yellow sandstone rock, full of a 

 quartzose conglomerate, and a perfect lumachelle of Exogyra virgula, 

 covered by a layer crowded with Trigonia Pellati and T. Munieri, 

 and then passing into 4 feet more of ferruginous conglomeratic grit 

 with the same fossils. These are Nos. 23-25 of M. Hebert. Seeking 

 these on the Portel side, we find the crystalline grit attached here 

 and there to the base of the great conglomerate, but very irregularly ; 

 towards the north it sinks by degrees till, opposite the Port du Mont 

 Couple, it is on the sea-shore. Here it encounters a considerable 

 fault, running nearly parallel to the cliff, and just cutting behind the 

 nearest cliff-quarries of Chatillon*. It is thus thrown to the top of 

 the cliff, and forms the base or perhaps the greater part of the 

 materials worked there, being in both places crystalline and ligni- 

 tiferous, and overlying the shales more or less unconformably. 

 Above it comes a variable mass of sands and clays, with huge grit 



* I at first thought the appearances were due to a local erosion and an over- 

 lap of the Chatillon beds ; but MM. Sauvage and Kigaux suggested a fault, 

 the direction of which was made clear by the observations of Prof. Prestwich 

 during an excursion of the Geological Society of France in September 1880. 



