228 THE WONDERS OF GEOLOGY. Lect. III. 



of the London and Paris basins, and the underlying chalk 

 and wealden deposits. 



19. The Paris Basin. — The Paris basin is from east 

 to west about 100 miles in extent, and 180 from north-east 

 to south-west ; the greatest total thickness of the beds, or 

 in other terms, the depth passed through to reach the chalk, 

 amounts to several hundred feet. 



The strata, commencing with the lowermost, or most 

 ancient, present the following characters : — 



1. The lowermost. Chalk flints, broken, and partially rolled, some- 



times conglomerated into ferruginous breccia.* 



2. Plastic clay, and sand. Strata of clay and sand, with fresh- 



water shells, drifted wood, lignite, leaves, and fruits; with 

 bands of limestone containing marine shells. 



3. Siliceous limestone, or Calcaire grossier ; with fresh- water 



and terrestrial shells and plants, and a coarse, marine, com- 

 pact limestone, passing into calcareous sand, and abound- 

 ing in marine shells. — These beds often alternate, and are 

 considered by M. Constant Prevost to be contemporaneous 

 formations; the marine strata having been formed in those 

 parts of the basin which were open to the sea, and the fresh- 

 water limestone, by mineral waters poured into the bay from 

 the south ; the Continent being situated then, as now, to the 

 south, and the ocean to the north. Layers of miliolite lime- 

 stone,! almost entirely composed of microscopic shells of 

 foraminifera, occur in this part of the basin. 



4. Gypseous marls, and limestones; with bones of terrestrial ani- 



mals, and' fluviatile shells. These are supposed to have been 

 discharged by a river which flowed into the gulf ; the gypsum 

 being precipitated from water holding sulphate of lime in solu- 

 tion, in the same manner as the travertine or calcareous tufa, of 

 which we have already spoken (p. 74). 



5. Upper marine formation, consisting of marls, micaceous and 



quartzose sand, with beds of sandstone abounding in marine 

 shells. 



* A layer of this kind is very common on our South Downs, imme- 

 diately under the turf. 



t So called from its inclosing immense quantities of a minute shell, 

 named Miliola. 



