Ch. Vm.] CHRONOLOGY OF EOCKS. 85 



oscillations of level, I have endeavored to show in my description of 

 that country ; * and the freshwater shells of existing species and hones 

 of land quadrupeds, partly of extinct races, preserved in the terraces 

 of fluviatile origin, attest the exclusion of the sea during the whole 

 process of filling up and partial re-excavation. 



Such terraces are the converse of those mentioned at p. 80, fig. 99, 

 where the uppermost of the series is formed of alluvium of oldest 

 date, which originated long before the valley had attained its actual 

 width and depth. 



CHAPTER VIII. 



CHRONOLOGICAL CLASSIFICATION OF ROCKS. 



Aqueous, plutonic, volcanic, and metamorphic rocks, considered chronologically — 

 Lehman's division into primitive and secondary — Werner's addition of a tran- 

 sition class — Neptunian theory — Hutton on igneous origin of granite — How the 

 name of primary was still retained for granite — The term "transition," why 

 faulty — The adherence to the old chronological nomenclature retarded the 

 progress of geology — New hypothesis intended to reconcile the igneous origin 

 of granite to the notion of its high antiquity — Explanation of the chronological 

 nomenclature adopted in this work, so far as regards primary, secondary, and 

 tertiary periods. 



In the first chapter it was stated that the four great classes of rocks, 

 the aqueous, the volcanic, the plutonic, and the metamorphic, would 

 each he considered not only in reference to their mineral characters, 

 and mode of origin, but also to their relative age. In regard to the 

 aqueous rocks, we have already seen that they are stratified, that 

 some are calcareous, others argillaceous or siliceous, some made up 

 of sand, others of pebbles ; that some contain freshwater, others ma- 

 rine fossils, and so forth; but the student has still to learn which 

 rocks, exhibiting some or all of these characters, have originated at 

 one period of the earth's history, and which at another. 



To determine this point in reference to the fossiliferous formations 

 is more easy than in any other class, and it is therefore the most con- 

 venient and natural method to begin by establishing a chronology for 

 these strata, and then to refer as far as possible to the same divisions 

 the several groups of plutonic, volcanic, and metamorphic rocks. 

 Such a system of classification is not only recommended by its great- 

 er clearness and facility of application, but is also best fitted to 

 strike the imagination by bringing into one view the contempora- 

 neous revolutions of the inorganic and organic creations of former 

 times. For the sedimentary formations are most readily distin- 

 guished by the different species of fossil annuals and plants which 



* Second Visit to the U. S., vol. ii., chap. 34. 



