14 INTRODUCTION. 



Of these primary groups, the Laurentian, Cambrian, Silu- 

 rian, Devonian, Carboniferous, and Permian are collectively 

 grouped together under the name of Primary or Paleozoic 

 Rocks (Gr. palaios, ancient ; zoe, life), because of the entire 

 divergence of their animals and plants from any now existing 

 upon the globe. The Triassic, Jurassic, and Cretaceous 

 systems are grouped together as the Secondary or Mesozoic 

 formations (Gr. mesos, intermediate; zoe, life), because their 

 organic remains are intermediate between those of the Pal- 

 aeozoic period, and those of more modern strata. The Eocene, 

 Miocene, Pliocene, and Post-tertiary Rocks are grouped together 

 under the head of Tertiary or Kainozoic Rocks (Gr. kainos, new; 

 zoe, life), because their organic remains approximate in char- 

 acter to those now existing upon the globe. 



CHAPTER III. 



CONTEMPORANEITY OF STRATA AND 

 GEOLOGICAL CONTINUITY. 



WHEN groups of beds in different parts of the earth's surface, 

 however widely separated from one another, contain the same 

 fossils, or rather an assemblage of fossils in which many iden- 

 tical forms occur, they are ordinarily said to be " contempora- 

 neous ; " that is to say, they are ordinarily supposed to belong 

 to the same geological period, and to have been formed at the 

 same time in the history of the earth. They would therefore 

 be unhesitatingly regarded as " geological equivalents/' and 

 would be classed as Silurian, Devonian, Carboniferous, and so 

 on. It is to be remembered, however, that it is not necessary, 

 to establish such a degree of equivalency between widely 

 separated groups of strata, that the fossils of each should be 

 to any great extent specifically identical. It is sufficient that, 

 whilst some few species are identical in both, the majority of 

 the fossils should be "representative forms," or, in other words, 

 nearly allied species. It will be shown, however, that groups 

 of strata widely removed from one another in point of distance 

 can only exceptionally be " contemporaneous," in the strict 

 sense of this term. On the contrary, in so far as we can judge 

 from the known facts of the present distribution of living beings, 

 the occurrence of exactly the same fossils in beds far removed 

 from one another is prima facie evidence, that the strata are 



