92 On the Geographical Extent of [Feb. 



traced only in some parts by the plastic clay which separates the 

 siliceous lime-stone from the chalk. 



3. Second marine Formation. — The strata of the third series 

 (etage) have a disposition different from those of the two preced- 

 ing ; but before 1 describe it, I should state the geological extent 

 which I attribute to this series. 



MM. Cuvier and Brongniart, with the clearness and precision 

 which they have shown throughout their work, have distin- 

 guished and characterized three particular sets of strata between 

 the two fresh-water formations ; namely, the marine marls above 

 the gypsum, the sands and sand-stones without shells, and the 

 upper sands and sand-stones with shells. Now, in considering 

 these strata in a general manner, I think that we may perceive 

 a large formation constituting the second marine epocha of the 

 Paris basin. In fact, the sands and the sand-stones without 

 shells are placed between two sets of beds, of which the fossils 

 are nearly similar. The upper one is of the same nature as the 

 strata without shells, which is here distinguished only by this 

 negative character. The lower set of beds is not so different 

 from it as it appears to be at first sight, since we know that a 

 bed belonging to any formation may be of a calcareous or a 

 quartzeous nature, according to its situation. This is actually 

 the case in this instance, as at Etampes ; the sand-stones and 

 grits without shells rest immediately on a sandy deposit contain- 

 ing many shells, among which are distinguished a great number 

 of pectunculi, cytherea;, See. ; that is to say, the same fossils as 

 are found in the sand that in other parts covers the grit without 

 shells. Lastly, the absence of organic remains in this latter 

 mass is only a repetition of a fact of frequent occurrence, namely, 

 that the mollusques diminish in numbers, and even disappear 

 entirely, in liquids that deposit siliceous matters.* 



The second marine formation, limited in this manner, is not 

 so concentrated as the first fresh-water formation. It begins to 

 appear sooner towards the north 5 but on the right bank of the 

 Seine it in general is seen only in thin parcels. It is in greater 

 masses to the south of that river, constituting the escarpments, 

 capes, and islands (Hot), which form the characteristic feature* 

 of the country extending towards Fontainebleau, Versailles, 

 Epernon, &c. Probably the facility with which the sandy strata 

 yield to the action of water is the cause of this physical disposi- 

 tion, by the denudation of the mass of sand that reached from 

 these escarpments to the detached parcels on the right bank. 



I do not know that the sandy strata have been observed at a 

 greater height than 200 metres above the sea. They follow the 

 general rule of inclining towards the south, and dip under the 

 second fresh-water formation, disappearing totally to the south of 

 a line drawn from Chartres to Nemours. 



• We adopt entirely this opinion of the author on the extent of this second marine 

 formation.— French Editor. 



