230 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



been found anywhere in the Paris district, although it would appear 

 to have extended across the Channel to the coast of Normandy. For 

 last April, in company with several members of this Society, we 

 examined the well-known cliff-section at the Lighthouse of Ailly near 

 Dieppe, and the majority of our party came to the conclusion that 

 the upper beds there belonged to the London Clay proper*. I had, 

 on a former visit, suspected the presence of the true London Clay, 

 but had found no organic remains to corroborate this opinion. On 

 this occasion, however, we were fortunate enough to discover a few 

 well-marked fossils, which, taken together with superposition and 

 mineral character, leaves little doubt in my mind of the existence of 

 the London Clay at that place. 



The chalk here rises about 80 feet in the cliff and is overlaid 

 by about 110 feet of Tertiary stata, the lower 60 of which belong to 

 the Woolwich and Reading series perfectly well characterized and 

 composed of sands, laminated clays, and pebble beds, with numerous 

 fluviatile and freshwater shells, on the ordinary Woolwich type, 

 forming a group very similar to that of the section at Castle Hill cliff, 

 Newhaven (see Quart. Journ. Geol. Soc. vol. x. p. 83). Above these 

 beds there is about 50 feet of laminated brown clay interstratified in 

 its lower part with several thin layers of sand, — a lithological character 

 of which we have occasionally some indications in the London Clay 

 on the opposite coast, for at White Cliff Bay the London Clay also con- 

 tains several subordinate beds of sand. This clay contains also some 

 iron pyrites and small light brown calcareo-argillaceous concretions. 

 At the base (which, by the by, is not very well defined) of this deposit 

 I found a few fragments of the Ditrupa plana and teeth of Lamnce, 

 so characteristic of the base of the London Clay in the Isle of 

 Wight and London districtf . Mr. A. Tylor, further, was fortunate 

 enough to find two small fossil crabs exactly in the condition in which 

 they occur in the London Clay, and which appear identical with a 

 species of Zanthopsis (probably a young specimen, the Z. nodosa) of 

 Potter's Bar and other places near London. There is no appearance 

 of any of the fossils of the " Lits Coquilliers." The London Clay 

 cannot be traced further in the direction of Paris, as the chalk is 



* The distinctiveness of these beds had not, however, escaped the practised eye 

 of M. d'Archiac, who has, I find, stated, so far back as 1839, that " the London 

 Clay formed the upper part of the cliff at the phare d' Ailly." He goes on to 

 say, however, that these clays are "similar to those of the Barton Cliffs, but we 

 have found neither fossils nor septaria." Bull. Soc. Geol. vol. x. p. 195, and Hist, des 

 Prog. vol. ii. p. 499. At the time this was written, the Barton Clays were con- 

 sidered to be the type of the London Clay. Although these two deposits are now 

 separated, this indication of M. d'Archiac is important, but has been generally 

 overlooked, for other French geologists, both before and since, have referred all 

 these beds to the sands and clays of the " Argile Plastique." 



t M. Hebert is, however, of opinion that some similar fossils equally well mark 

 a thin conglomerate bed at the base of the Calcaire grossier. The " Dentalium 

 strangulatum " may certainly prove to be synonymous with the Ditrupa plana ; 

 but, if so, it ranges up to the " Sables moyens " and is no longer characteristic of 

 a particular zone. The specific characters of the teeth of Lamncb are also ex- 

 tremely problematical. To these two fossils alone I should attach no great weight. 

 The prevalence of closely allied species might result from like conditions tending 

 to the recurrence of analogous forms of life at distant periods of time. 



