200 



On Formations, 



[MarcHj 



one siliceous, the otlier calcareous. The siliceous mineral is 

 sometioies like flint, sometimes Vike. jasper, and at times it is 

 vesicular, like buhrsione. The limestone is sometimes compact, 

 sometimes marly; often contains irregular cylindric cavities, 

 nearly parallel, though crooked. They resemble exactly the 

 cavities that would be formed in a bed of mud by bubbles of gas 

 rising from the bottom to the surface. This limestone, when 

 fresh gathered from the quarry, has often the property of disin- 

 tegrating by the iiifluence of the air and water, and hence is 

 used as marl. But a principal character of this formation is the 

 presence of fresh water shells : these are different species of 

 helix, planorbro, limneus, potamides, cyclostoma^ gyrogonites^ 

 and bulimus. 



li. Alluvial. — The alluvial, or uppermost formation, is 

 composed of variously coloured sand, marl, clay, or a mixture 

 .of these substances impregnated with carbon, which gives the 

 mixture a brown or black colour. It contains rolled stones of 

 different kinds, but is most particularly characterised by contain- 

 ing the remains of large organic bodies. It is in this formation 

 that we find large trunks of trees, bones of elephants, also 

 of oxen, rein-deer, and other large mammalia. This alluvial 

 ' matter is deposited in hollows that have been scooped out of the 

 solid rocks we have just enumerated. It is a very old deposite, 

 as it appears to have been formed before the commencement of 

 our history, because it contains remains of trees and animals 

 different from any that exist at present in the neighbouring coun- 

 try, or in the globe. 



From the preceding account it would appear that the strata 

 around Paris are of clay, gravel, , sand, sandstone, millstone or 

 buhrstone, marl, limestone, chalk, and gypsum ; and the^e are 

 said to constitute 1 1 different formations. It would probably 

 simplify our view of this tract of country,, and be equally correct, 

 if we diminished the number of formations in the following 

 2iianner :— 



1. Chalk formation. 2. Coarse marine limestone formaiiony 

 under which we would include not only the coarse limestone, but 

 also the siliceous limestone, plastic clay, and sand, because this 

 latter is intermixed with the limestone, and there is an uninter- 

 rupted transition from the one into the other. 3. Gypstim 

 formation, or first fresh water formation. 4. Sandstone jorma^ 

 tion, under which might be included the sand and sandstone 

 without shells, the upper marine sandstone, and the buhrstone- 

 without shells. 5, Second fresh ivater formation^ composed of 

 limestone and flint. 6, Alluvial j'ormaiion. 



From the intermixture of fresh and salt water, organic pro- 

 ductions ir) these formations, we may suppose that both tiiese 

 jBuids must have contributed each ^heir part in their formation. 



