CONTEMPORANEITY AND HOMOTAXIS. 53 



on the borders of an ocean, in the neighbourhood of land, differ 

 wholly in mineral character from the peculiar " muds " formed in 

 the profound depths of the same ocean ; an equal difference existing 

 as to the character of the animals buried in each. We thus learn 

 that marine deposits may be strictly contemporaneous, and may be 

 placed near to one another in point of distance, and yet may be 

 wholly unlike both lithologically and zoologically. Lastly, synchron- 

 ous deposits necessarily contain wholly different fossils, if one has 

 been deposited by fresh water and the other has been laid down in 

 the sea. The fresh-water deposits of one period are obviously con- 

 temporaneous with the marine formations of the same period, and 

 they may not be far removed from one another in point of distance, 

 but they must contain altogether different organic remains. The 

 former will contain remains of the fresh-water and terrestrial animals 

 of the period, and of these only ; whilst the latter will principally, if 

 not exclusively, be characterised by the remains of marine forms of 

 life. In this way, there is reason to believe, may be explained the 

 differences between the fossils of the Old Red Sandstone and of the 

 Devonian rocks, strictly so called. Both are believed to have been 

 deposited in the same geological period, and to be truly " contem- 

 poraneous " ; but they do not contain the same fossils. This may 

 be readily explained, however, if we suppose the former to represent 

 the fresh-water deposits of the Devonian period, or to have been laid 

 down in an inland sea, whilst the latter is the true marine formation 

 of the same period. 



In the strictest sense, then, of the term, deposits may be spoken 

 of as " homotaxial " when they contain identical or closely allied 

 fossils, but have nevertheless not been laid down at precisely the 

 same time. If such deposits are widely separated from one another 

 in space, then the possession of identical fossils is a direct argu- 

 ment in favour of a want of absolute contemporaneity — suppos- 

 ing that the deposits compared have been formed under similar 

 conditions, in which case alone a complete comparison is possible. 

 Thus, the Lower Carboniferous rocks of Britain, Russia, China, 

 and North America are all marine in origin ; and the fact that 

 they contain identical species of Brachiopods is thus an argument 

 in favour of the view that they were not formed at precisely the 

 same time, since they are so widely apart that they cannot be re- 

 garded as having been simultaneously laid down within the limits of 

 a single ocean. Nevertheless, the deposits in question were laid 

 down in the same geological period, and are therefore " geological 

 equivalents." The doctrine of " homotaxis," therefore, if rightly 

 limited and denned, in no way diminishes the value of fossils as indi- 

 cative of the age of the formations in which they occur. If we 

 give the term " contemporaneous " a purely geological sense, and 



