TIIK .ITIIASSIC SVSTKM 457 



With respect to the limestones, it is noticeable that they are 

 t h in- 1 >t ( hied and compact and mostly of an earthy or argillaceous 

 natmv, so that they must have been originally more or less 

 eah-aivnus muds. Hence it is probable that the calcareous matter 

 had a chemical or sedimentary rather than an organic origin. Mr. 

 II. I 1 .. Wmxhvanl thinks they may have been derived mechanically 

 from the waste of Palzeozoic limestones, 25 but it is more likely that 

 the conditions were favourable to the chemical precipitation of 

 carbonate of lime from solution. 



The geographic arrangement of land and sea in Liassic time was 

 at first only a modification of that which prevailed in Keuper time 

 (see Fig. 124) ; but the sea gradually extended itself over a larger 

 area. There is no evidence that the Lower Lias occurs under the 

 northern part of the Paris basin, and there is no trace of it in the 

 Boulonnais. In Normandy it only occupied what may be called 

 the buy of Carentan, and does not occur near Caen. The Middle 

 Lias, however, extends much farther south, patches of it being found 

 at intervals in the Departments of Calvados, Orne, Sarthe, and 

 Maine, while Upper Lias has been reached in a deep boring at 

 Rouen. Thus it appears that by the time of the Middle Lias a 

 channel of communication had been established between the British 

 Sea and that which spread over the greater part of France. 



There were, however, two large islands in this western part of 

 the Liassic Sea. One of these was the central plateau of France ; 

 proof of this is found in the fact that the Charmouthian deposits, 

 in the form of sands and conglomerates, overlap the Sinemurian at 

 many places round its borders, and rest directly on the Archiean 

 rocks. Similarly in Poitou, near Thouars, the Toarcian in its turn 

 passes beyond the Charmouthian as a consequence of continued 

 subsidence. The other island lay farther north; it comprised a part 

 of Eastern England and the southern part of the North Sea, with 

 the north-east of France and the whole of Belgium. 



The Liassic Sea seems to have occupied the whole of Germany 

 and Central Europe, as well as Italy and the greater part of Spain, 

 and it extended eastward through Hungary to the Carpathian 

 Mountains, and through Transylvania to Kronstadt, but it does 

 not seem to have reached so far east in Northern Europe, for no 

 Liassic deposits have yet been found in Poland or Western Russia. 



Qlevanian Time. In the west of Europe and in those places 

 where the succession of marine deposits is most complete, as in the 

 south-west of England, the north of France, and the Jura Mountains, 

 there is a more or less rapid change from Liassic Clays through sands 

 to limestones. There is also a considerable change in the fauna, 

 and it is evident that the sea-water became clear where it had been 





