EVIDENCE IN STRATIGRAPHICAL GEOLOGY. 45 



a particular stage in chronological succession, while the fresh-water or 

 terrestrial deposits of the same series through their vegetable remains 

 would indicate another, it is to the former that we should give the 

 precedence as determining the age of the entire series. Such cases 

 are sometimes spoken of as instances of " homotaxis," but this is not 

 strictly so. In the particular cases here in question, two sets of beds, 

 which may have been formed contemporaneously, are found to con- 

 tain fossils of apparently different geological ages, in consequence of 

 the fact that the beds have been formed under different conditions, the 

 one containing the remains of land-plants, and the other the remains 

 of marine Invertebrates. The apparent difference of age is due to 

 the fact that the evolution of the land-flora in the particular region 

 where the beds are found has not been parallel with that of the 

 marine fauna of the same region. On the other hand, " homo- 

 taxial " deposits, properly so called, are deposits which have been 

 formed during the same geological period, and have been laid down 

 under similar conditions, thus coming to contain similar classes of 

 fossils, but which have been formed in regions very far apart. The 

 similarity, or identity, of the fossils in the two sets of beds proves 

 them to belong to the same general period ; but their geographical 

 remoteness is a proof that they were formed at different stages of 

 this period, and that they were not precisely identical. Together 

 with the similarity of certain types of life in " homotaxial " beds, 

 there is found a dissimilarity as regards other types, this being a 

 consequence of the fact that the two sets of beds have been formed 

 in widely distant areas, and therefore in distinct zoological provinces. 

 In other words, the dissimilarity in the fossils in " homotaxial " beds, 

 in the strict sense of the term, is dependent on the distance in space 

 of the beds, and is not due to difference of origin. 



An excellent concrete example of the above general principle is afforded 

 by the so-called "Dakota Beds" of North America. These are largely 

 developed in the basin of the Upper Missouri, are mainly of brackish- 

 water or fresh-water origin, and contain a series of plant-remains the 

 general aspect of which is clearly Tertiary. If judged, therefore, by 

 purely palaeobotanical evidence, the " Dakota Beds " would be assigned 

 to the Eocene system. The "Dakota Beds" are, however, overlain by 

 some thousands of feet of stratigraphically younger deposits charged 

 with marine fossils of Cretaceous type. Judged, therefore, by a palseo- 

 zoological standard, the " Dakota Beds " must be assigned to the Cre- 

 taceous period. The explanation of this discrepancy in the age of the 

 beds as deduced from the plants and animals respectively is apparently 

 twofold. On the one hand, the "Dakota Beds" are mainly of fresh- 

 water origin, whereas the strata by which they are surmounted were laid 

 down in the sea. On the other hand, we must suppose that the Tertiary 

 flora had been introduced into the American area at a time when the 

 seas of the same area were still tenanted by the characteristic animals of 

 the Cretaceous period, and that the latter were not replaced by the ani- 

 mals distinctive of the Tertiary period until long after the land-vegeta- 



