S96 ■*' On the relative Age of the 



chalk system — The tertiary formations — Lastly, the first or old 

 transported or alluvial deposites *. 



Although all these formations have been dejxjsited by the 

 waters, although they are met with in the same localities, and 

 upon each other, the transition from the one to the next species 

 does not take place by insensible gradations. A sudden and 

 abrupt variation is always observed in the physical nature of the 

 deposite, and in that of the organized beings, whose remains 

 Occur in them. Thus, it is evident, that between the epoch at 

 which the Jura limestone was deposited, and thai of the preci- 

 pitation of the green sand and chalk system which covers it, a 

 complete renovation in the state of things took place at the sur- 

 face of the globe. The same may be said of the epoch which 

 separated the precipitation of the chalk from that of the tertiary 

 forinations, as it is equally manifest that in each place the state 

 in which the nature of the fluid from which the formations were 

 precipitated must have completely changed between the time of 

 the tertiary formation and that of the old transported or alluvial 

 formations. 



• \yith llic object which I liavc in view, an accurale definition of these 

 formations is unnecessary. I might even liave contented myself with nam- 

 ing and designating them as Nos. 1, 2, 3, 4. No. 1 would have been, for ex- 

 ample, the oldest sedimentary I'ormation of the four, that which the others 

 cover, iu a word, the Jura limestone. No. 4 would then be Ibund connected 

 with the superior formation, or that of the transported dcjjosites. I shall, 

 however, give a very brief account of these deposites, in as far as regards their 

 nature and aspect. 



M. de Humboldt has given the name of Jura limestone to the vast sedimen- 

 tary deposite of wliich the Jura Mountains are in a great measure composed, 

 an,d, which is formed by a whitish limestone, sometimes compact and uniform, 

 like the lithographic limestone which is extracted from it, sometimes com- 

 posed of small round grains named Oolites, whence the designation of oolilic 

 iimestotic. 



The secUmentary Ibrmaliou comprising the grccnsand and the c/ia/Ar, c(m- 

 ^s>is of a succession of sandstone beds, often mixed with a large quantity of 

 small green grains of silicate of protoxide of iron, and surmounted by a very 

 thick series of beds of 'lialk. The beds of both sjiecies which form the cliffs 

 of the English Channel are the type of this kind of formation. 



The tcrliari/ scdimeulary formation is that of the neighbourhood of Paris. It 

 is a very varied succession of beds of clay, limestone, marl, gypsum, sand- 

 stone, and buhrstonc. 



Lastl}', The old alluvial formations derive their name from tlicir resemblance 

 to the alluvia produced by the rivers of the present period. 



