154 ELEMENTS OF BIOLOGY. 



Subjoined is a table giving the more important subdivi- 

 sions of the three great geological periods, commencing 

 with the oldest rocks and ascending to the present day. 

 (See fig. 39.) 



I. Palaeozoic or Primary Rocks. 



1. Laurentian. (Lower and Upper.) 



2. Cambrian. (Lower and Upper, with Huronian Rocks?) 



3. Silurian. (Lower and Upper.) 



4. Devonian, or Old Red Sandstone. (Lower, Middle, 

 and Upper.) 



5. Carboniferous. (Mountain-limestone, Millstone Grit, 

 and Coal-measures.) 



6. Permian. ( = The lower portion of the New Red 

 Sandstone.) 



IL Mesozoic or Secondary Rocks. ' 



7. Triassic Rocks. (Bunter Sandstein, or Lower Trias ; 

 Muschelkalk, or Middle Trias; Keuper, or Upper Trias.) 



8. Jurassic Rocks. (Lias, Inferior Oolite, Great Oolite, 

 Oxford Clay, Coral Rag, Kimmeridge Clay, Portland Stone, 

 Purbeck beds.) 



9. Cretaceous Rooks. (Wealden, Lower Greensand, 

 Gault, Upper Greensand, White Chalk, Maestricht beds.) 



IIL Kainozoic or Tertiary Rocks. 



10. Eocene. (Lower, Middle, and Upper.) 



11. Miocene. (Lower and Upper.) 



12. Pliocene. (Older Pliocene and Newer Pliocene.) 



13. Post-tertiary. (Post-pliocene and Recent.) 



« 



Contemporaneity of Strata. — When groups of beds 

 in different regions contain the same fossils, or rather an 

 assemblage of fossils in which many identical forms occur, 

 they are ordinarily said to be "contemporaneous;" that 

 is to say, they are ordinarily supposed to have been formed 



m 



