20 THE MEDALS OF CREATION. Cuap. I. 
loose earthy deposit, to compact volcanic lava, may be traced 
in numerous instances, so as to leave but little doubt that 
the rocks called primitive or primary, may have originally 
been either argillaceous, siliceous, or calcareous strata, abound- 
ing in organic remains (Wond. p. 873). These crystalline 
masses have been formed at successive periods ; for granite 
is found of all ages, occurring in the most ancient, as well as 
in comparatively modern epochs. The difference between 
the composition and aspect of these rocks, and those of 
recent volcanoes, is with much probability ascribed to 
the fact that the latter are of sub-aerial origin ; that is, 
were erupted on the surface, and the gaseous products in 
consequence escaped ; while the former were ejected at great 
depths, either beneath the sea, or under immense accumu- 
lations of other deposits, and being thus subjected to great 
pressure, the volatile elements were confined, and formed — 
new combinations: in like manner as chalk when burnt in 
the open air is converted into lime, the carbonic acid gas 
escaping ; but when exposed to the same degree of heat 
in a closed iron tube, is transformed into granular marble 
(Wond. p. 104). 
From these ancient crystalline rocks generally underlying 
the sedimentary deposits, and never appearing as if they 
had been ejected from a crater, the term hypogene* (nether- 
formed) is employed by Sir C. Lyell to designate the whole 
class ; and they are subdivided into, 1. plutonic, those in 
which all traces of sedimentary origin are lost, as granite ; 
and 2. metamorphic, those which still manifest traces of stra- 
tification, as mica-schist, We. 
The fossiliferous rocks are, for the convenience of study, 
separated into three grand divisions. 
* Nether-formed, from vr, hypo, under; and yivoua:, ginomaz, to 
be formed. 
