PLUTONIC OE IGNEOUS ROCKS. 643 



CHAPTER IL 



PLUTONIC OR IGNEOUS ROCKS. 



LUTO, presiding over fire, was early in 

 troduced into the nomenclature of geo 

 logy, to supply a generic title for those 

 vast masses which consist of melted 

 mineral matter cooled down, which have 

 been driven up from the interior deep- 

 seated regions of the globe, from whence 

 the lavas, scoriae, and ashes of the modern 

 epoch are erupted through the volcanic 

 vents. Recurring to former pages of this 

 work, some of the evidence will be found 

 recorded in favour of the doctrine, that 

 the interior shell of the solid crust upon 

 which we tread is in contact with an 

 immensely high temperature, which per 

 meates through it, gradually diminishing 

 with the distance from the focus of heat ; 

 and that at a certain extent below the 

 surface of the earth, perhaps not more 

 than thirty miles, matter exists in a state 

 of fusion. According to some eminent 

 geologists, the volcanic action that marks 

 the present history of our planet, is the 

 comparatively feebler play of identically 

 the same causes, which, in the earlier part of its story, operated with inconceivable energy 

 in the structure and configuration of its frame-work. It must be admitted, that nothing 

 analogous to the productions of ancient time the granites and porphyries can now be 

 seen in the course of formation upon the actual surface of the globe ; yet we can only 

 be acquainted with part of the workmanship of the volcano with the products ejected 

 whose diverse character is no proof that granite is not elaborated in the depths of the 

 furnace. Be this as it may, the evidence is irresistible of the intense action of heat in 

 the construction and elevation of those rocks to which the term Plutonic or igneous has 

 been attached, and in the transformation or metamorphic change of the primary strata. 

 Sir C. Lyell distinguishes such rocks by the term Hypogenous, from inro and yivopa.^ " pro 

 duced under," or "nether-formed ;" but such an appellation is scarcely distinctive, for it 

 will equally apply to the sedimentary strata, constructed and consolidated beneath the 

 waters of the ocean. There is no valid reason in this case for changing a well-known 

 nomenclature, which, in every science, should be as little disturbed as possible. 



The igneous rocks have been variously classified by Dr. Macculloch, M. Brongniart, 

 and Mr. Scrope, according to their mineralogical composition ; but a more general, though 

 less scientific, arrangement is prevalent. This refers to the different aspects of igneous 

 products, which have resulted more from the varying circumstances under which cooling 

 down and solidification have taken place, than from any original and real difference of 



