ELEMENTS OF GEOLOGY. 



PART II. 



CHAPTER XII. 



ON THE DIFFERENT AGES OF THE FOUR GREAT CLASSES OF ROCKS. 



Aqueoua, plutonic, volcanic, and metamorphic rocks, considered chronologi- 

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

 transition 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 pro- 

 gress of geology — New hypothesis invented to reconcile the igneous origin of 

 granite to the notion of its high antiquity — Explanation of the chronological no- 

 menclature adopted in this work, so far as regards primary, secondary, and ter- 

 tiary periods. 



In the first part of this work the four great classes of rocks, 

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

 have been considered with reference to their external characters, 

 their mineral composition, and mode of origin ; and it now re- 

 mains to treat of the same classes with reference to the different 

 periods at which they were formed. In speaking of the aqueous 

 rocks, for example, it has been shown that they are stratified, 

 that some are calcareous, others argillaceous, some made up of 

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

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

 which rocks, exhibiting some or all of these characters, have ori- 

 ginated at one period of the earth's history, and which at an- 

 other. 



So in regard to the volcanic and plutonic formations, we have 

 hitherto examined their mineral peculiarities, forms, and mode 

 of origin, but have still to inquire into their chronological his- 

 tory. 



Lastly, a more curious question will dennand our attention, 

 when we endeavour to ascertain the relative ages of the meta- 

 morphic rocks, the chronology of which may be said to be two- 

 fold, each formation having been deposited at one period, and 

 having assumed a crystalline texture at another. 



