THE STRUCTURE AND CLASSIFICATION OP KOCKS. 637 



In addition to this amount of animal matter, the vegetable kingdom has contributed, in 

 an inferior degree, yet in no unimportant manner, to the composition of strata, as the 

 anthracite, coal, and lignite formations attest, which occupy extensive areas of country. 



The thickness of the fossiliferous rocks in Great Britain is thus given by Professor 

 Phillips : 



Tertiary strata, including the \ n-nf 



diluvial clay, and sand. J 

 Chalk - - 600 



Green-sand ... 480 



Wealden group ... 900 



Oolite mean thickness - - 1230 



Lias - 1050 



New red sandstone - - DOO 



Magnesian limestone - - 300 feet. 



Coal measures ... 30OO 



Millstone grit ... 900 



Carboniferous limestone - - 180O 



Old red sandstone ... 99OO 



Silurian system ... 7470 



Cambrian system - - 90OO 



The total, according to this estimate, is 34,080 feet, or about 6-5 miles : but it appears from 

 more recent investigations, that the Cambrian group and Silurian system far exceed in 

 thickness the entire aggregate of the other strata. 



The division of rocks into fossiliferous and non-fossiliferous, into stratified andunstratified, 

 is founded upon obvious and important natural characters, and accordingly all geologists are 

 here agreed. Nor is there any difference of opinion as to the order of succession and 

 relative age of the larger formations, however, in the details of classification, the grouping 

 of strata, and the distinctive terms employed, some diversity prevails, perplexing to the 

 student. The first arrangement Avas made by Lehman, who divided all rocks into 



Primitive. Those of hard and slaty structure, containing no fossil organic remains. 



Secondary. Rocks of comminuted fragments, and containing organic remains. 



Local. Those of partial occurrence in different districts. 



Werner introduced another class between the primary and secondary, to which he applied 

 the term Transition, from the indications they exhibited of a transition state from the one 

 to the other as to mineral structure, and of having been formed Avhen the world Avas passing 

 from an uninhabitable to a habitable condition. Afterwards the word Local gave place 

 to Tertiary, Avhich became the generic title of all regularly stratified beds above the chalk 

 up to the superficial formations ; and the transition class was incorporated Avith the 

 secondary. But as geology advanced, this arrangement was soon found to be far too 

 general. Each of these grand divisions has since been subdivided into systems and groups 

 of strata, which are so distinguished from each other by mineral or fossil peculiarities, as 

 to justify the idea that they were formed independently, under different conditions, during 

 successive epochs. Thus, beginning at the surface, we have, in descending order, the 

 following systems, distributed into great life periods, according to natural history dis 

 tinctions : 



CLASSIFICATION OF STRATIFIED ROCKS. 



Cainozoic Period f 1. Post-Tertiary System. 

 (Recent Life.) \ 2. Tertiary System. 



Mesozoic Period J' Cretaceous, or Chalk System. 



4. Oolitic System. 



5. Upper New Red Sandstone, or Triassic System. 



Lower New Red Sandstone, or Permian System. 



Palaeozoic Period 

 (Ancient Life.) 



7. Carboniferous System. 

 Old Red Sandstone System. 



9. Silurian System. 



Azoic Period ( 10 - CIa y- slate s y stem - 



(Void of Life.) iJJ- Mica-Schist System. 

 9 J ' ( 12. Gneiss System. 



This arrangement is more clearly exhibited on the next page. 



