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GEOLOGICAL MEMOIRS. 



Keuper gradually diminishes in thickness. It certainly once more 

 makes its appearance, leaning against the sandstones of Central 

 France, in Cher-Department, and at the Canal du Centre ; but far 

 from such sandstone beds it is reduced to a minimum ; an arkose 

 rock takes its place at last (Avallon), and it now disappears entirely, 

 so that the Lias rests upon granite. The Keuper and Trias, there- 

 fore, disappear entirely from the Bourbonnais to England. In Cal- 

 vados and Sarthe-Department, the boundary-conditions are very inter- 

 esting ; here the blue Arietes-limestones lie immediately upon the 

 red Transition sandstones with Trilobites, facts which may be ob- 

 served between Caen and Alencon at many places in the quarries 

 near the road. The line of distinction between these two closely ad- 

 jacent formations is so absolute, that one can knock off hand-speci- 

 mens consisting half of Trilobite sandstone, half of blue limestone 

 with Lias fossils. A remarkable appearance seen here is, that pebbles 

 of the sandstone are inclosed in the ^n'e£es-limestones, and even in 

 the younger calcareous marls of the middle Lias lying above the 

 Arietes-be&s : and, on the other hand, the Lias mud has penetrated 

 into the clefts and. cracks of the sandstone, and there become 

 hardened. 



The extent and divisions of the Lias are determined by its bounda- 

 ries. Sometimes the Lias forms only a small strip, closely border- 

 ing the steep sides of the mountains, and sometimes it extends 

 into plains ; the different strata of the under, middle, and upper 

 Lias are often arranged one upon another regularly in a terrace-like 

 form ; and often the terraces appear as if they were drawn away 

 from each other, the different strata lying near each other at the 

 same height. Here must the Swabian Jura-rocks be taken as the 

 normal type, for in no other country are the bedding of the strata 

 and the terrace formation better marked. In Swabia sometimes the 

 Lias has a plain-like extension, according to the development of the 

 Keuper boundary (as between Eyach and Schlichem, Tubingen and 

 Stuttgart, Rems and Kocher, Ellwangen and Offingen) ; sometimes 

 it presents only narrow strips (as at the Wutacb, Upper Neckar, 

 between Hechingen and Reutlingen) ; but everywhere we soon 

 arrive at a correct result, because the separate strata are mostly 

 distinguishable in their outlines and the gradations of the ground. 

 But this is quite different as soon as we pass the Rhine, because 

 from thence the terrace-like arrangement of the beds one upon 

 another disappears. In the Jura, or in the lower land of Berne, 

 where the "black Jura" is developed, there is no great extension 

 in width ; there are mostly slips or rents where the different 

 strata show themselves in profile in a mass of about 100'; at the 

 bottom of the rents the fossils of all the strata lie mixed together. 

 The position of the strata one by the side of another is also the case 

 in Burgundy, the Cher-Department, and the Calvados. In passing, 

 for instance, from Avallon to Vassy, where the celebrated " Roman 

 cement" is manufactured from the Posidonomyce-schists, or from Arcy 

 to Avallon, all the Lias series is found exposed near the roads which 

 traverse a plain, the distinctive limits of the strata having been re- 

 moved ; and the observer passes imperceptibly from stratum to stratum. 



