54 INTRODUCTION. 



endeavour to forget its literal signification as applying to events 

 which have occurred at precisely the same moment of time, then it is 

 just as good an epithet for the different deposits belonging to a given 

 geological formation as is the term " homotaxial." All the de- 

 posits which possess Carboniferous fossils, at whatever point of the 

 earth's surface they may be situated, belong to the Carboniferous 

 period, and are therefore geologically contemporaneous. All that is 

 really implied by the doctrine of " homotaxis," rightly regarded, is 

 that we cannot say that any great formation in any one country is 

 the precise equivalent of the same formation in any country very 

 widely removed in point of distance, in the sense that its deposition 

 began and ended at exactly the same times ; and therefore we can- 

 not parallel the subdivisions of such formations with anything 

 approaching to absolute precision. Regarded as a whole, however, 

 the Carboniferous formation of America is the geological equivalent 

 of the Carboniferous formation of Europe, and both belong to what 

 geologists understand as the " Carboniferous period." As the same 

 is true of all the great formations, in all parts of the world, it is clear 

 that the principal advantage of the use of such a term as " homo- 

 taxis " is simply that we thereby avoid the employment of a word 

 which common usage would wrongly interpret ; and it is quite cer- 

 tain that we cannot abolish the idea of geological "contemporaneity,' 7 

 as demonstrated by the presence of identical or representative species 

 of fossils ; nor can we refuse to admit that formations containing 

 such fossils, however far removed from one another in point of dis- 

 tance, must have been laid down within the limits of the same great 

 " period " in the history of our earth. 



In the case of the Lower Carboniferous deposits above alluded to, not 

 only are the fossils generally similar to one another in all the widely sep- 

 arated regions in which these strata have been recognised, but certain 

 characteristic species seem to have an almost universal distribution. The 

 enormous range of certain specific types remains a most noteworthy fact, 

 whatever explanation may be given of it. As a general rule, however, 

 deposits in different geographical areas, which occupy a corresponding 

 position in the series of the stratified rocks, and are therefore geo- 

 logically speaking "contemporaneous," contain but few or no actually 

 identical species. The faunae of such deposits, on the contrary, are usu- 

 ally composed of allied or representative forms, the same groups of ani- 

 mals being the ones prevalent in and characteristic of the deposits, but the 

 species being different. Thus, there can be no doubt that the Devonian 

 deposits of North America and Western Europe are geological equiv- 

 alents, since they occupy in each region the interval between the highest 

 Silurian and the lowest Carboniferous sediments. The fauna of the 

 Devonian rocks of North America, also, presents a close general resem- 

 blance to that of the Devonian of Devonshire, Belgium, and Germany. 

 In both cases there is a predominance of certain special groups of ani- 

 mals, amongst which the great Hydrozoal group of the Stromatoporoids 

 may be particularly singled out. The Stromatoporoids of the American 



