176 



MINERALS AND GEOLOGY 



opposed to the view of igneous fusion ; and yet quartz of the sai 

 character does occur sparingly in many trachytes, and under condi- 

 tions not favourable to the idea that it may have been subsequently 

 introduced by aqueous agencies. Through these trachytes, moreover, 

 there is a gradual passage into actual lavas or known fusion-products;, 

 whilst, on the other hand, many syenites (containing free quartz) 

 merge gradually into greenstone and basalt, products intimately 

 related to augitic lavas. It is, of course, impossible to say in what 

 form a rock belonging now to the eruptive .class may actually have 

 originated. It may have been produced from, an earlier formed 

 igneous or crystalline mass, or from a sedimentary deposit buried 

 deeply under overlying beds. The endogenous or subterranean 

 agencies, whatever they may have been, that rendered granite and 

 syenite plastic and crystalline, also produced the crystalline texture 

 and other related characters of gneiss, mica schist, hornblende rock, 

 and other members of the metamorphic series. It is now very 

 generally assumed that whilst ordinary lavas and most trachytes and 

 trappean (or basaltic) rocks have solidified from a molten condition, 

 other rocks of this class, the granites and syenites more especially, 

 have been rendered plastic and crystalline by "hydro-igneous" 

 agency. These rocks, in other words, are thought to have undergone 

 a kind of aqueous fusion and subsequent crystallization, the water, 

 originally present in them, having been retained for a time by the 

 pressure exerted on the plastic mass at great depths. But this view, 

 it must be understood, is entirely hypothetical, and in many respecta 

 is far from satisfactory. All that is really known may be thus 

 expressed : Two sets of forces are concerned, either alone or con 

 jointly, in the production of rock masses generally. One set, entirely 

 external, or consisting essentially in the action of the atmosphere 

 and waters on the surface of the earth-mass, produces the sedimentary 

 or stratified rocks proper. The other forces, of internal or subter- 

 ranean origin, produce the unstratified rocks, as we now see these 

 latter, and lead to the crystallization and metamorphism of sedimen- 

 tary strata brought within their influence. But whether granites, 

 syenites, traps, and trachytes, be igneous or non-igneous rocks, they 

 are evidently related products, and members of a common class. 



These rocks are arranged by Sir Charles Lyell in two broad 

 divisions : Volcanic and Plutonic rocks ; but it is impossible to draw 



