Ch. XIV.] SLOWNESS OF DEPOSITION. 231 



Hints ; and in the limestone we meet with shells of Limnca, PlanorbU, 

 and other lacustrine genera. 



Proofs of gradual deposition. — Some sections of the foliated marls in 

 the valley of the Cer, near Anrillac, attest, in the most unequivocal man- 

 ner, the extreme slowness with which the materials of the lacustrine series 

 were amassed. In the hill of Barrat, for example, we find an assemblage 

 of calcareous and siliceous marls ; in which, for a depth of at least 60 

 feet, the layers are so thin, that thirty are sometimes contained in the 

 thickness of an inch ; and when they are separated, we see preserved in 

 every one of them the flattened stems of Charce, or other plants, or some- 

 times myriads of small Paludince and other freshwater shells. These 

 minute foliations of the marl resemble precisely some of the recent lamina- 

 ted beds of the Scotch marl lakes, and may be compared to the pages of 

 a book, each containing a history of a certain period of the past. The 

 different layers may be grouped together in beds from a foot to a foot 

 and a half in thickness, which are distinguished by differences of composi- 

 tion and color, the tints being white, green, and brown. Occasionally 

 there is a parting layer of pure flint, or of black carbonaceous vegetable 

 matter, about an inch thick, or of white pulverulent marl. We find sev- 

 eral hills in the neighborhood of Aurillac composed of such materials, for 

 the height of more than 200 feet from their base^ the whole sometimes 

 covered by rocky currents of trachytic or basaltic lava.'* 



Thus wonderfully minute are the separate parts of which some of the 

 most massive geological monuments are made up ! "When we desire to 

 classify, it is necessary to contemplate entire groups of strata in the aggre- 

 gate ; but if we wish to understand the mode of their formation, and to 

 explain their origin, we must think only of the minute subdivisions of 

 which each mass is composed. We must bear in mind how many thin 

 leaf-like seams of matter, each containing the remains of myriads of tes- 

 tacea and plants, frequently enter into the composition of a single stratum, 

 and how vast a succession of these strata unite to form a single group ! 

 We must remember, also, that piles of volcanic matter, like the Plomb 

 du Cantal, which rises in the immediate neighborhood of Aurillac, are 

 themselves equally the result of successive accumulation, consisting of 

 reiterated sheets of lava, showers of scoriae, and ejected fragments of 

 rock. — Lastly, we must not forget that continents and mountain-chains, 

 colossal as are their dimensions, are nothing more than an assemblage of 

 many such igneous and aqueous groups, formed in succession during an 

 indefinite lapse of ages, and superimposed upon each other. 



Miocene strata of Bordeaux and South of France. — A great extent 

 of country between the Pyrenees and the Gironde is overspread by 

 tertiary deposits of various ages and chiefly of Miocene date. M. 

 Tournouer, in an able memoir on these formations,! has shown that 



* Lyell and Murchisou, Sur les DepOts Lacustres Tertiaires du Cantal, &c. Ann. 

 des Sci. Nat., Oct, 1829. 



f Bulletin Soc. Geol. de France, tome xviii., 1861-2, p. 1035. 



