Oft I] FOUR CLASSES OF ROCKS CONTEMPORANEOUS. 9 



that the aqueous and volcanic rocks were afterwards superimposed, and 

 should, therefore, rank as secondaiy in the order of time. This idea 

 was adopted in the infancy of the science, when all formations, whether 

 stratified or unstratified, earthy or crystalline, with or without fossils, 

 were alike regarded as of aqueous origin. At that period it was natu- 

 rally argued, that the foundation must be older than the superstructure ; 

 but it was afterwards discovered, that this opinion was by no means in 

 every instance a legitimate deduction from facts ; for the inferior parts 

 of the earth's crust have often been modified, and even entirely changed, 

 by the influence of volcanic and other subterranean causes, while super- 

 imposed formations have not been in the slightest degree altered. In 

 other words, the destroying and renovating processes have given birth 

 to new rocks below, while those above, whether crystalline or fossilif- 

 erous, have remained in their ancient condition. Even in cities, such as 

 Venice and Amsterdam, it cannot be laid down as universally true, that 

 the upper parts of each edifice, whether of brick or marble, are more 

 modern than the foundations on which they rest, for these often consist 

 of wooden piles, which may have rotted and been replaced one after 

 the other, without the least injury to the buildings above ; meanwhile, 

 these may have required scarcely any repair, and may have been con- 

 stantly inhabited. So it is with the habitable surface of our globe, in 

 its relation to large masses of rock immediately below : it may continue 

 the same for ages, while subjacent materials, at 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 viewed as be- 

 longing 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 second- 

 ary, the terms primitive and primary, which were formerly used for the 

 whole, must be abandoned, as they 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 pecu- 

 liarity 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 unal- 

 tered sedimentary strata. I proposed in the Principles of Geology (first 

 edition, vol. iii.), the term "hypogene" for this purpose, derived from 

 otfo, under, and /ivofAcu, 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. They occupy 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 

 modem date, belonging, for example, to the period hereafter to be 

 described as tertiary, they are still underlying rocks. They never repose 



