20 



LYELL'S ELEMENTS OF GEOLOGY. 



Plutonic Rocks and their Origin. 



Plutonic rocks. — We have now therefore pointed out the ex- 

 istence of two distinct orders of mineral masses, the aqueous and 

 the volcanic: but if we examine a large portion of a continent, 

 especially if it contain within it a lofty mountain range, we rarely 

 fail to discover two other classes of rocks, very distinct from 

 either of those above alluded to, and which we can neither assimi- 

 late to deposits such as are now accumulated in lakes or seas, 

 nor to those generated by ordinary volcanic action. The mem- 

 bers of both these divisions of rocks agree in being highly crys- 

 talline and destitute of organic remains. The rocks of one divi- 

 sion have been called plutonic, comprehending all the granites 

 and certain porphyries, which are nearly allied in some of their 

 characters to volcanic formations. The members of the other 

 class are stratified and often slat}^, and have been called by some 

 the crystalline-schists. In these are included gneiss, micaceous- 

 schist (or mica-slate,) hornblende-schist, statuary marble, the 

 finer kinds of roofing slate, and other rocks afterwards to be 

 described. 



As it is admitted that nothing strictly analogous to these crys- 

 talline productions can now be seen in the progress of formation 

 on the earth's surface, it will naturally be asked, on what data 

 we can find a place for them in a system of classification founded 

 on the origin of rocks. First then, in regard to the plutonic 

 class, a passage has been traced from various kinds of granite 

 into different varieties of rocks decidedly volcanic ; so that if the 

 latter are of igneous origin, it is scarcely possible to refuse to 

 admit that the granites are so likewise. Secondly, large masses 

 of granite are found to send forth dikes and veins into the con- 

 tiguous strata, very much in the same way as lava and volcanic 

 matter penetrate aqueous deposits, both the massive granite and 

 the veins causing changes analogous to those which lava and 

 volcanic gases are known to produce. But the plutonic rocks 

 differ from the volcanic, not only by their more crystaUine tex- 

 ture, but also by the absence of tuffs and breccias, which are the 

 products of eruptions at the earth's surface. They differ also by 

 the absence of pores or cellular cavities, which the entangled gases 

 give rise to in ordinary lava. From these and other peculiari- 

 ties it has been inferred, that the granites have been formed at 

 great depths in the earth, and have cooled and crystallized slowly 

 under enormous pressure where the contained gases could not 

 expand. The volcanic rocks, on the contrary, although they 

 also have risen up from below, have cooled from a melted state 

 more rapidly upon or near the surface. From this hypothesis 

 of the great depth at which the granites originated, has been de- 

 rived the name of " Plutonic rocks," which they have received 



