As all the crystalline rocks may, in some respects, be viewed as belonging to one great family, whether they be stratified or unstratified, plutonic or metamorphic, it will often be convenient to speak of them by one common name. It being now ascertained, as above stated, that they are of very different ages, sometimes newer than the strata called secondary, the term primary, which was formerly used for the whole, must be abandoned, as it would imply a manifest contradiction. It is indispensable, therefore, to find a new name, one which must not be of chronological import, and must express, on the one hand, some peculiarity equally attributable to granite and gneiss (to the plutonic as well as the _altered_ rocks), and, on the other, must have reference to characters in which those rocks differ, both from the volcanic and from the _unaltered_ sedimentary strata. I proposed in the Principles of Geology (first edition, vol. iii.), the term "hypogene" for this purpose, derived from +hypo+, _under_, and +ginomai+, _to be_, or _to be born_; a word implying the theory that granite, gneiss, and the other crystalline formations are alike _nether-formed_ rocks, or rocks which have not assumed their present form and structure at the surface. This occurs in the lowest place in the order of superposition. Even in regions such as the Alps, where some masses of granite and gneiss can be shown to be of comparatively modern date, belonging, for example, to the period hereafter to be described as tertiary, they are still _underlying_ rocks. They never repose on the volcanic or trappean formations, nor on strata containing organic remains. They are _hypogene_, as "being under" all the rest. From what has now been said, the reader will understand that each of the four great classes of rocks may be studied under two distinct points of view; first, they may be studied simply as mineral masses deriving their origin from particular causes, and having a certain composition, form, and position in the earth's crust, or other characters both positive and negative, such as the presence or absence of organic remains. In the second place, the rocks of each class may be viewed as a grand chronological series of monuments, attesting a succession of events in the former history of the globe and its living inhabitants. I shall accordingly proceed to treat of each family of rocks; first, in reference to those characters which are not chronological, and then in particular relation to the several periods when they were formed. FOOTNOTES: [3-A] See Principles of Geology, by the Author, Index, "Nile," "Rivers," &c. [4-A] See p. 18. [4-B] See Geograph. Journ. vol. iv. p. 64. CHAPTER II. AQUEOUS ROCKS--THEIR COMPOSITION AND FORMS OF STRATIFICATION. Mineral composition of strata--Arenaceous rocks--Argillaceous--Calcareous--Gypsum--Forms of stratification--Original horizontality--Thinning out--Diagonal arrangement--Ripple mark.