POUR CLASSES OF ROCKS. 
33 
earth. Intensely heated water or steam permeating strati¬ 
fied masses under great pressure have no doubt played 
their part in producing the crystalline texture and other 
changes, and it is clear that the transforming influence has 
often pervaded entire mountain masses of strata. 
In accordance with the hypothesis above alluded to, I pro¬ 
posed in the first edition of the Principles of Geology ( 1833 ) 
the term “ Metamorphic ” for the altered strata, a term de¬ 
rived from fiETa, meta, trans, and fjioptprj, morphe, forma. 
Hence there are four great classes of rocks considered in 
reference to their origin—the aqueous, the volcanic, the plu- 
tonic, and the metamorphic. In the course of this work it 
will be shown that portions of each of these four distinct 
classes have originated at many successive periods. They 
have all been produced contemporaneously, and may even 
now be in the progress of formation on a large scale. It is 
not true, as was formerly supposed, that all granites, togeth¬ 
er with the crystalline or metamorphic strata, were first 
formed, and therefore entitled to be called primitive,” and 
that the aqueous and volcanic rocks were afterwards super¬ 
imposed, and should, therefore, rank as secondary in the or¬ 
der of time. This idea was adopted in the infancy of the 
science, when all formations, whether stratified or unstrati¬ 
fied, earthy or crystalline, with or without fossils, were alike 
regarded as of aqueous origin. At that period it was nat¬ 
urally 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 de¬ 
duction 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 
superimposed formations have not been in the slightest de¬ 
gree altered. In other words, the destroying and renovating 
processes have given birth to new rocks below, while those 
above, whether crystalline or fossiliferous, have remained in 
their ancient condition. Even in cities, such as Venice and 
Amsterdam, it can not 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 Avhich they rest, 
for these often consist of wooden piles, which may have rot¬ 
ted 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 constantly 
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 
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