The sedimentary origin ascribed to ancient crystalline rocks is, of course, hypothetical, since all evidence of aqueous deposition is wanting, and the minerals (_mica_, _quartz_, and _felspar_) of which they are so largely constituted, are not readily soluble in water under ordinary circumstances. But rocks unquestionably deposited by water, when exposed to intense heat under great pressure, acquire a crystalline structure (_Wond._ p. 864); and a series of changes, from a 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, abounding 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 accumulations 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_[10] (_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 stratification, as mica-schist, &c. [10] Nether-formed, from ιπο, _hypo_, under; and γἱνομαι, ginomai, to be formed. The fossiliferous rocks are, for the convenience of study, separated into three grand divisions. 1. The Tertiary; comprising the deposits between the Chalk and the superficial Drift and modern Alluvium. 2. The Secondary; from the _Chalk_ to the _Trias or New Red_, inclusive. 3. The Palæozoic; from the _Permian_ to the _Silurian_; including the vast series of unfossiliferous slate rocks termed the _Cambrian_, in which all traces of organic remains are lost.