34 
ELEMEISITS OE GEOLOGY. 
a great depth, are passing from a solid to a fluid state, and 
then reconsolidating, so as to acquire a new texture. 
As all the crystalline rocks may, in some respects, be view¬ 
ed as belonging to one great family, whether they be strati¬ 
fied or unstratified, metamorphic or plutonic, it will often be 
convenient to speak of them by one common name. It be¬ 
ing now ascertained, as above stated, that they are of very 
different ages, sometimes newer than the strata called sec¬ 
ondary, the terms primitive and primary which were former¬ 
ly used for the whole must be abandoned, as they would im¬ 
ply 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 refer¬ 
ence to characters in which those rocks differ, both from the 
volcanic and from the unaltered sedimentary strata. I pro¬ 
posed in the Principles of Geology (first edition, vol. iii.) the 
term “ hypogene ” for this purpose, derived from vtto, under^ 
and ylvojjLai^to he^ or to he horn; a word implying the theory 
that granite, gneiss, and the other crystalline formations are 
alike netherformed rocks, or rocks which have not assumed 
their present form and structure at the surface. They occu¬ 
py 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, be¬ 
longing, for example, to the period hereafter to be described 
as tertiary, they are still underlying rocks. They never re¬ 
pose 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 maybe studied 
simply as mineral masses deriving their origin from particu¬ 
lar causes, and having a certain composition, form, and posi¬ 
tion in the earth’s crust, or other characters both positive 
and negative, such as the presence or absence of organic re¬ 
mains. In the second place, the rocks of each class may be 
viewed as a grand chronological series of monuments, attest¬ 
ing 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. 
