Missing Chapters of Geological History. 21 



that the comparison of the molluscous remains from this part of 

 the series was found so far to agree with previous determina- 

 tions with regard to fish remains, as to demand its distinct 

 classification with New Eed Sandstone rocks. These beds are 

 now recognized at the base of the Lias from Lyme Regis into 

 the south of Leicestershire, and they may exist further towards 

 Yorkshire, though hitherto they have not been described. 

 They are very uniform in positiou, mineral character, and 

 fossil contents, and have yielded upwards of sixty species. 

 All these species differ from Muschelkalk species, but they are 

 mostly of the same genera, and the aspect of the group is 

 similar ; while, on the other hand, without any lithological or 

 stratigraphical break, there is only one species in common, 

 between this bone bed and the lower Lias shales. There is, 

 thus, a clear palasontological break between the bone bed and 

 the lower Lias. The representative beds corresponding to this 

 break are not yet discovered. 



The Lias in England is a remarkably uniform deposit, con- 

 sisting of muddy rocks converted into shales, and separated by 

 bands more or less completely calcareous. The middle division, 

 called the Marlstone, is the most like a limestone, and the 

 uppermost part of the upper division is sandy. 



Palaeontologists have endeavoured to separate the Lias into 

 a number of sub-divisions by the Ammonites, groups of species 

 of these shells being characteristic of different zones. The 

 evidence on this point rests on the assumption of specific dif- 

 ferences being indicated by permanent modifications of the 

 structure of the shell. But it is quite possible that these may 

 mean nothing more than would be due to some change in the 

 conditions of existence. Except between the Marlstone and 

 the upper Lias there is really no palaeontological break in the 

 proper sense of the words. Alteration of form and size, conse- 

 quent on the occurrence of circumstances more or less favour- 

 able, migration of species, and other well known causes, suf- 

 ficiently account for many of those modifications of the form of 

 the shell that have been taken as specific marks. This view 

 is strengthened by the fact, that the other shells and other 

 organisms generally show no proof of a break of any im- 

 portance, except at the point already alluded to. 



The Oolites overlie the Lias, but there is no clear and 

 decided break between the upper Lias sands and the bottom beds 

 of the inferior Oolite. This is well seen near Cheltenham, where 

 passage beds occur. Bearing in mind that there is a consider- 

 able change of mineral character, the number of species (one- 

 fifth of the whole) that pass from the upper Lias into the 

 inferior Oolite is large, and the results of comparison of the 

 other molluscous remains point to the same conclusion. 



