220 



LONDON CLAY. 



2. Lower Freshwater Beds. \ Sometimes intermixed with Ma- 



( Fine. 



a Marl. ' 

 b Gypsum. 



3. Upper Marine Formation. 



a Sand, Sandstone and Millstone without Shells. 

 h Sandstone with Shells. 



4. Upper Freshwater Formation. 



b 8)"" Millstone " - } ^eshwater Shells. 



The tertiary strata supposed to be more recent, and called Qua- 

 ternary, are nowhere observed covering the above formations, be- 

 cause they were deposited in detached seas or lakes: the evidence 

 of these being more recent than the strata in the J^aris and London 

 basins, rests on the opinion, that the species of shells which they con- 

 tain, are, in a large proportion, analogous to existing species. 



Plastic Clay and London Clay. — These, with the various asso- 

 ciated beds of sand, may properly be regarded as one formation, of 

 which the plastic clay is the lowest member, resting on chalk. Near 

 Paris, the plastic clay is a very thin bed ; but in the south of France 

 it acquires a great degree of thickness, and appears to comprise the 

 upper argillaceous beds, or what we call the London clay : it is re- 

 markable for the vegetable fossils and beds of lignite, which it fre- 

 quently, but not invariably, contains. In England, in the lower beds 

 of this formation, there are found beds of imperfect wood coal ; but 

 both in the plastic clay and the London clay, remains of marine ani- 

 mals are chiefly prevalent, though intermixed with some freshwater 

 shells ; whereas, on the Continent, beside the great quantities of fos- 

 sil wood and wood coal found in the same argillaceous beds, there 

 are numerous remains of freshwater shells, which render their title 

 to be denominated marine formations more than doubtful. The beds 

 of sand are sometimes of considerable thickness. By many geolo- 

 gists it is maintained that the beds of soft sandstone (called Molasse,) 

 and of sandstone conglomerate (called Nagelflne^ in Switzerland,) 

 belong to this part of the tertiary formations. That some of these 

 beds may be tertiary 1 will not deny ; but I am fully convinced, that 

 many beds called molasse, in Savoy, are covered by the Jura lime- 

 stone and oolites, having repeatedly seen them in contact, and got 

 specimens from each bed at the line of junction.* 



* As the opinions of geolo^-ists have been much divided respecting the molasse, 

 or soft sandstone, of Switzerland and Savoy, I shall here insert some observations 

 wpon it, given in the first volume of my Travels in the Tarentaise. 



" The outer calcareous mountains on the western side of Savoy, all rest upon an 

 immense formation of soft sandstone (molasse,) and are interstratified with it; 

 and, so far from this sandstone being more recent than the limestone (as Saussure 

 supposed,) it constitutes a considerable part of the bulk of these mountains that are 



