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other animals well known on the banks of the river. Although 

 the bird itself has not been seen, there can be no doubt of its 

 existence, as the impression was found in the soft bank in the 

 middle of the river, which a heavy storm would obliterate, amongst 

 other well-known species. The trees in the dirt-bed in the neigh- 

 bourhood of Weymouth stand in or rest upon the soil in which 

 they grew ; consequently their submersion beneath the water in 

 which the Purbeck beds were formed, and which now cover 

 them, must have been gradual and unaccompanied by a rush of 

 waters. Such is the case in the present formation of their equi- 

 valent in the lagoons of Santa Martha, where the trees are grow- 

 ing in shallow water, subject to slight oscillations above and be- 

 low the level of the sea. Sometimes the salt water prevails, and 

 the sediment becomes thickly covered with marine shells : during 

 the great floods pouring down the Magdalena, carrying with it 

 immense forests and the debris of rocks, the lagoons become again 

 covered with freshwater and terrestrial organic remains. Thus, 

 by variable seasons, and by the slow oscillation of the land by sub- 

 terranean forces acting for a long period, the variable sedimen- 

 tary beds are formed, such as we now see them when exposed to 

 view. Lacustrine deposits of the same period may present very 

 different characters, depending on the nature of the locality in 

 which they may be formed: for instance, the lagoons of the 

 Orinoco may contain a very different series of organic remains, 

 as compared with those of the Magdalena; hence formations 

 may be of the same age and the same zone, yet different in de- 

 grees of induration and organic contents, as well as in the rela- 

 tive geological position. Viewed generally over a considerable 

 area, the rock called chalk is based either upon an arenaceous 

 or argillaceous deposit. As we cannot expect uniformity in the 

 distribution of detritus, unless under conditions which could 

 scarcely ever obtain, we should anticipate that sands would pre- 

 dominate over one part of the area and mud or silt over another, 

 as we now find to be the case in the present deposits forming 

 within the tropics. The white variety may probably have re- 

 sulted from the precipitation of carbonate of lime from solution 

 in water, in a favourable locality, suitable for the reception of 

 calcareous springs. 



The supracretaceous group of Europe completes the great sedi- 



