1854.] HAMILTON MAYENCE BASIN. 287 



although he refuses to entertain the idea of connecting these divisions 

 with sudden and violent interruptions in the ordinary course of 

 events, or with violent revolutions in the physical geography of the 

 earth's surface, he cannot deny that the changes of strata are owing 

 to physical causes and to alternate elevations and depressions occa- 

 sioned by subterranean convulsions. If, then, these subdivisions of 

 Eocene, Miocene, and Pliocene are to be considered as representing 

 true geological horizons, it must be admitted that they depend on 

 some external facts, some natural, but partial convulsions, which, by 

 altering the relative levels of land and water, or by other similar 

 causes, changed the conditions of life, causing the destruction of some 

 then existing species, and producing the necessary conditions for the 

 introduction or creation of new ones. If this be not the case, the 

 lines or divisions are made to depend on the accidental varieties of 

 numbers, which may be owing to merely local or temporary causes, 

 and could hardly be considered as a sufficient basis for a philosophi- 

 cally-established nomenclature. 



It is, therefore, not unreasonable to assume, that, wherever the 

 real line of demarcation between any two subdivisions is to be drawn, 

 some physical change or convulsion must have occurred at no great 

 distance, by which the conditions of life were suddenly altered, 

 although further oif no real break took place in the continuity of 

 deposits, and there the change of species was so gradual, and almost 

 imperceptible, that it was impossible to draw a true geological line of 

 separation. Where, however, the sudden change did take place, there 

 the line of demarcation can be distinctly drawn, and will thus serve 

 as a correct guide to the separation of other beds. We have, then, 

 no difficulty in saying, where one deposit ceases, " This is the end of 

 the old system," and where a new deposit commences, " This is the 

 beginning of the new system." Now, this is precisely the position 

 we are in with regard to the Weinheim beds of the Mayence basin. 

 They have been satisfactorily shown to be identical with the Middle 

 Limburg beds ; the Middle Limburg beds are part of a continuous 

 series, gradually merging into those below and those above. They 

 are called indiscriminately Upper Eocene by one author. Lower Mio- 

 cene by another ; and, belonging partially to both, they can yet be 

 fully claimed by neither. With the Weinheim beds the case is very 

 different ; they can be shown, under successive modifications, clearly 

 to belong to the same system as the beds above, but they have no 

 relation whatever to those below. They lie unconformably on the 

 Carboniferous Sandstone ; they form the commencement of a series, 

 the basis of the whole system. Here, then, we are enabled to 

 say, " This is the beginning of the new system ;" and we have con- 

 sequently no ditRcult)^ and ought to have no hesitation, in looking 

 on the Middle Limburg, as well as the Weinheim beds, as the com- 

 mencement of the Miocene series. 



It would, I think, be inconsistent with all analogy to call a bed, 

 which unequivocally forms the commencement of the whole tertiary 

 system in the country, the upper bed of an old expiring series, rather 



