THE STRUCTURE AND CLASSIFICATION OP ROCKS. 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! , 1350 feet 



diluvial clay, and sand. J 



Chalk - - 600 



Green-sand - 480 



Wealden group - - 900 



Oolite mean thickness - - 1230 



Lias - 1050 



New red sandstone - - 900 



Magnesian limestone - 300 feet. 



Coal measures - - 3000 



Millstone grit - 900 



Carboniferous limestone - - 1800 



Old red sandstone - 9900 



Silurian system ... 7470 



Cambrian system ... 9000 



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 and Silurian systems, the study of which 

 is of recent date, 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 was 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 when the world was passing 

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

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

 up to the superficial formations. 



Upon comparing the remains exhibited by formations of different ages together, 

 decisive evidence appears of the progressive development of organic life, from the more 

 simple and imperfect structures that obtain in the older strata, to the higher organisations 

 that are found in those depositions which belong to aeras immediately antecedent to the 

 existing epoch. In the course of our planet's history, the less complex tribes of animals 

 and plants were the first to appear, the more perfect species becoming more and more 

 numerous up to the creation of the present races. But this view is widely different 

 from that theory of gradual perfectionnement in the same species of animals and plants 

 which some writers have adopted, and which the evidence afforded by organic remains 

 amply contradicts. It is true that fishes, which of all vertebrata rank the lowest, appear 

 first in geological history; but then they are not imperfect formations they have no 

 mark of inferior organisation, but occur in their highest state of approximation to the 

 reptile, and not in their lowest condition of affinity to the worm. It is also true, that 

 reptiles precede mammalia, but they are reptiles belonging to the higher grades of that 

 class. The earliest zoophytes and molluscae likewise display no inferiority in their 

 organisation when compared with their living representatives. Hence, the right con- 

 clusion is, that while there has been a progressive development of organic life upon the 

 face of the globe, it has not been by an improvement of species, but by an addition of 

 fresh organisations, a new dramatis persona, to meet new physical conditions of the earth. 



