CHAPTER XI 11. 

 OF IGNEOUS HOCKS. 



IGNEOUS rocks, as wo have already stated, are those rocks which 

 have resulted from the consolidation of molten magmas. The 

 evidence of this mode of origin, so far as regards the volcanic 

 rocks, is complete and satisfactory. Molten masses are seen to 

 issue from volcanoes and to consolidate as more or less crystalline 

 rocks; moreover, it is possible, as wo shall presently show when we 

 describe the remarkable synthetic experiments of Messrs. FOUQUE and 

 LEVY to reproduce such rocks as basalt, andesite and tephrite by pure 

 igneous fusion. When, however, we pass from the consideration of the 

 volcanic to that of the plutonic rocks the evidence is not so satisfactory. 

 Granite, diorite and gabbro have never been seen to form at the surface 

 of the earth, and all attempts to reproduce them artificially have 

 failed. Nevertheless, when we study the geological, chemical and 

 mineralogies! relations of the plutonic to the volcanic masses, and 

 note the transitional forms, we are driven to conclude that the former 

 have resulted from the crystallization of a mass of material similar to 

 that which has yielded the latter, and that the difference between the 

 two types of rock is a consequence of the fact that the consolidation 

 lias taken place under different conditions as to temperature and 

 pressure. 



The origin of plutonic masses is still further complicated by the 

 fact that they show a double kind of relationship. On the one hand 

 they are closely connected with volcanic rocks, and on the other with 

 the rocks which constitute the banded crystalline group. Those geolo- 

 gists who have regarded the banded gneisses as metamorphosed 

 sedimentary rocks have looked upon the massive rocks, associated 

 with them, as representing the extreme phase of metamorphism, 

 and have spoken of these massive rocks as metamorphic granite, 

 metapmorhic gabbro, &c., &c. That sedimentary rocks have occasionally 

 been melted down may be regarded as highly probable, though there 

 does not appear to be any direct evidence of this having taken place 

 on a large scale ; but that the crystalline schists, banded gneisses and 

 igneous rocks, taken as a whole, have resulted from the metamorphosis 



