﻿44 



GEOLOGICAL MEMOIRS. 



and reefs of corals are met with in the " brown Jurassic" rocks : in 

 Swabia they are rare. With the depth of the sea the size of the shells is 

 also related ; — the Terebratulce and Spiriferi of the Lias are twice as 

 large in Calvados as in Swabia ; the Ammonites of the ornati-c\ay 

 grow here to such a gigantic size, that one who has only seen the 

 Swabian specimens can scarcely recognize them again. We need not 

 point out that the materials also of the deposits, and their greater or 

 less thickness are connected with this point. 



3. The quality of the water. — Apart from the many springs and 

 currents in the sea which carry with them lime, quartz, and other 

 matters, to be contributed to the formation of rocks, there are espe- 

 cially two great causes which sometimes conjointly, and sometimes 

 in succession, are continually at work, contemporaneously forming a 

 stratum with varying characters at different places. These import- 

 ant factors are the salt-water and the river-water. The former is par- 

 ticularly adapted to form limestone beds, consisting of the, often 

 scarcely distinguishable, remnants of molluscs and zoophytes. Re- 

 mains of a few plants and bones, scattered about without order, bi- 

 valves 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 Sepiee 

 are common. The molluscs lie frequently grouped into families, in 

 nests together. Fixed corals are entirely wanting. Constant Prevost 

 has ventured to explain from these two causes alone the differences 

 of all the formations. Among the Jurassic strata he points out as of 

 marine formation the Arietes- and Grypkcete-beds, the oolitic lime- 

 stones of the "brown Jurassic" series, the Great Oolite, the macro- 

 cephali-hed, 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 cla}^. At all events we here 

 see how one stratum is variously formed in different localities, ac- 

 cording 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 the dif- 

 ferent strata. Most of the names are only correct 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 development. Why do they 

 still give to many a genuine German stratum an English name which 



* [See alsoMr.R.N. Mantell on the Oolite of Wilts, Quart. Journ. Geol. Soc. 

 vol. vi. p. 315. — Transl.] 



