GEOLOGICAL CONTINUITY. 55 



Devonian represent those of the European Devonian, and certain peculiar 

 generic types (such as Idiostroma) are common to both regions ; but 

 there is, nevertheless, a marked difference as regards the forms most 

 prevalent in each. Thus, the European Devonian is specially character- 

 ised by the prevalence of species of Actiiwstroma and Stromatopora, 

 while the most characteristic types of the American Devonian belong to 

 the genus Clathrodictyon, a genus which in Europe attains its maximum 

 in the Silurian period. Again, only few species of Stromatoporoids are 

 common to the two regions in question. Precisely similar facts could be 

 brought forward with regard to the Corals, Brachiopods, &c, of the De- 

 vonian formation in these areas ; and the general law which these facts 

 illustrate would seem to be one of very wide application. 



In other cases, while the general fauna of two geologically equivalent 

 deposits in widely remote areas is broadly similar, special organic types 

 may occur in one or other area which indicate a later or an earlier age 

 than that pointed to by the general assemblage of fossils. A marked 

 example of this is found in the occurrence of such characteristic Second- 

 ary types of Cephalopods as Am?no?iites and Ceratites in certain strata 

 in North-western India, the general fossils of which are of unequivocal 

 Carboniferous type (Waagen). In this case, special organic types which 

 are in general characteristic of a later age, are associated with a general 

 fauna distinctive of an earlier period. An instance of the opposite of this 

 — i.e., of the occurrence of a special organic form of an earlier period in 

 association with a general fauna of a later type — is found in the occur- 

 rence of Ammonites in California in strata containing abundant fossils 

 characteristic of the Eocene Tertiary. 



Geological Continuity. 



We are now in a position very briefly to discuss the question of 

 what may be called "geological continuity." It has already been 

 stated that the entire series of Fossiliferous or Sedimentary rocks 

 may be naturally divided into a certain number of definite rock- 

 groups or " formations," each of which is characterised by the 

 possession of a peculiar and characteristic assemblage of fossils, 

 constituting, or rather representing, the "life" of the "period" in 

 which the formation was deposited. The older geologists held, what 

 probably every one would be tempted to think at first, that the close 

 of each formation was characterised by a general destruction of the 

 forms of life of the period, and that the commencement of each new 

 formation was accompanied by the creation of a number of new 

 animals and plants, destined to figure as the characteristic fossils of 

 the same. This theory, however, not only invokes forces and pro- 

 cesses which it can in no way account for, but overlooks the fact 

 that most of the great formations are separated by lapses of time, 

 unrepresented perhaps by any deposition of rock, or represented 

 only in some particular area, and yet, perhaps, as great as, or 

 greater than, the whole time occupied in the production of the 

 formation itself. 



Upon any theory of evolution, however, it must be held that 



