Aqueous, plutonic, volcanic, and metamorphic rocks, considered chronologically--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 progress of geology--New hypothesis invented 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 be considered not only in reference to their mineral characters, and mode of origin, but also to their relative age. The preservation of the shelves may have required, says Darwin, the quick growth of green turf on a good soil; their abrupt cessation may mark the place where the soil was barren, and when a green sward formed slowly. 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 marine 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 convenient and natural method to begin by establishing a chronology for these fossiliferous strata, and then to endeavour to refer to the same divisions, the several groups of plutonic, volcanic, and metamorphic rocks. This system of classification is not only recommended by its greater clearness and facility of application, but is also best fitted to strike the imagination by bringing into one view the past changes of the inorganic world, and the contemporaneous revolutions of the organic creation. For the sedimentary formations of successive periods are most readily distinguished by the different species of fossil animals and plants which they inclose, and of which one race after another has flourished and then disappeared from the earth. But before entering specially on the subdivisions of the aqueous rocks arranged according to the order of time, it will be desirable to say a few words on the chronology of rocks in general, although in doing so we shall be unavoidably led to allude to some classes of phenomena which the beginner must not yet expect fully to comprehend.