of Local Influences upon the Formation of Strata, 109 



salt water and the river water. The former is particularly 

 adapted to form limestone beds, consisting of the often scarcely 

 distinguishable remnants of molluscs and zoophytes. Remains 

 of a few plants and bones, scattered about without order, 

 bivalves with their separated shells, and remains of other 

 molluscs lie buried, or rolled up in heaps, more or less water- 

 worn. But where river water unites itself with the sea, 

 there the deposits consist of clay, argillaceous limestones, 

 and sandstones, according to the nature of the soil through 

 which the rivers take their course. Regular strata here 

 exist. Remains of plants and wood ; saurii and sepise are 

 common. The molluscs lie frequently grouped into families, 

 in nests together. Fixed corals are entirely wanting. Con- 

 stant Prevost has ventured to explain from these two alone 

 the diiferences of all the formations. Among the Jurassic 

 strata he points out as of marine formation the arietes and 

 gryphcece beds, the oolitic limestones of the '* brown Jurassic" 

 series, the great oolite, the macrocephali bed, the coral rag, 

 and Portland limestone ; and as fluvio-marine formation, the 

 lias sandstone, the *' black Jurassic" clays and marls, the 

 sandstones and clays of the brown Jurassic series, the Oxford 

 clay,* and the Kimmeridge clay. At all events we here see 

 how one stratum is variously formed in different localities, 

 according to the local influence of the sea water, either by 

 itself or in connection with river water. 



If we regard the different Jura formations from this point 

 of view, we see also the impossibility of having common 

 names for different strata. Most of the names are only cor- 

 rect as local names, and have only a special meaning ; but 

 as soon as these special names are applied to other similar 

 strata, they confuse and puzzle us. What confusion have 

 the appellations Bradford clay, Oxford clay, Kimmeridge 

 clay, and particularly the Portland, already caused in the 

 German Jurassic formation % The Germans have indeed 

 a right to be proud of their Jura series, for in no other 

 country have these rocks found such clear stratigraphical 



* (See also Mr R. N. Mautell on the Oolite of Wilts. Quart. Journ. Geol. 

 Soc, vol. vi., p. 315.— Transl.) 



