842 



THE WONDERS OF GEOLOGY. Lect. VIII. 



action, we may reasonably infer that the play of electro- 

 chemical or electro-magnetic forces, which must be inces- 

 santly going on among the heterogeneous mineral substances 

 of which the earth consists, is fully adequate to produce the 

 varied effects we have been contemplating.* 



As the fragments of unmelted rock, which are occasionally 

 thrown up from the foci of volcanoes apparently of enormous 

 depth, consist of granite, quartzose porphyry, and the like, 

 some philosophers are of opinion that a primitive granitic 

 rock was the substratum, and is the support, of the super- 

 imposed sedimentary and fossiliferous strata. 



21. Hypogene Rocks. f — We must now enter upon a 

 more particular examination of the Hypoge?ie, or meta- 

 morphic and plutonic rocks ; those masses of crystalline 

 minerals, which everywhere manifest the influence of in- 

 tense heat under great pressure. It will be convenient to 

 consider them under two heads: viz. 1st, the rocks which 

 present a stratified or laminated structure, as mica-schist, 

 gneiss, &c. ; 2dly, those which occur in amorphous masses, 

 as granite, porphyry, &c. ; including, in this group, the 

 ancient volcanic product, trap or basalt, a notice of which 

 we found it requisite to anticipate, when investigating the 

 fossiliferous strata traversed by dikes of this substance. 



And here it is necessary to premise, that an acquaintance 

 with the nature and appearance of the minerals that are the 

 usual components of primary rocks, is indispensable to 

 enable the reader to have a clear conception of the facts 

 that will come under his notice. This knowledge can only 

 be acquired by the study of specimens ; and it were useless 

 to attempt by mere description to teach the elements of 



* The various theories that have been proposed to explain volcanic 

 action, are considered with great candour and perspicuity by Sir H. de 

 la Beche, in his charming volume entitled, Researches in Theoretical 

 Geology. 



f See ante, p. 20G. 



