74 RICIITUOFEN NATURAL SYSTEM 



certain foliated rocks, and as it appears no less safe to conclude that those rocks, the 

 intrusive or extrusive character of which is generally admitted, originated from be 1 

 low the sedimentary rocks, we have still to examine where is the limit to which the 

 theory of such an origin may he safely applied. A line of gradual passages connects 

 the foliated rocks with gneiss, and through it with granite, and another line of 

 gradual passages can be traced from the volcanic through the porphyritic to the 

 granitic, and through them again to the gneissoid rocks. The affinity of granite and 

 gneiss may therefore be said to be the central point from which all other lines of 

 passage in the nature of rocks diverge ; and it must be ascribed to this relation, 

 that many geologists, while admitting the origin from below the crust of the globe 

 of "trap-rocks" and "lava," still consider granite, s}'enite and cognate rocks to 

 have been generated by the metamorphism of sediments, and to occupy now the 

 same position which these sediments had before being metamorphosed. The pe- 

 culiar character of granitic texture, which is nearer allied to that of foliated crystal- 

 line than to that of volcanic rocks, the geological occurrence of granite in intimate 

 connection with those and its apparent independence of position in reference to the 

 latter, the singular order of solidification of the minerals constituting granitic 

 rocks — these and other reasons of a similar kind would indeed appear to support 

 the assumption that granitic and volcanic rocks differ in regard to their origin ; while, 

 on the other hand, additional probability is apparently given to the connection of 

 granite and foliated rocks in regard to their mode of origin, if it is taken into con- 

 sideration that many varieties of gneiss resemble granite closely in chemical com- 

 position, although others deviate from it in this respect and do not conform to the 

 law of Bunsen. 



According to the theory here advocated, all those rocks will be the true repre- 

 sentatives of the primordial mass of the globe, which are subject, in regard to their 

 composition, to the law of Bunsen, and occur, besides, in positions which place 

 beyond doubt their intrusive or extrusive origin, so far as they no longer occupy 

 their original place. As regards the first distinguishing mark, it is known that granite, 

 syenite, diorite, diabase, and nearly all those porphyritic rocks which have been con- 

 sidered as of hypogene origin are subject to the law of Bunsen, as far as their composi- 

 tion has been ascertained. In this respect, therefore, they are fundamentally different 

 from sedimentary and metamorphic rocks. Although believing this to be conclusive 

 evidence of their origin from below the shell of sediments, we must enter into a short 

 discussion of the chief objections which have been raised against the necessary corollary 

 from this mode of origin, which is the eruptive character of those portions of granite 

 which we see on the surface, and the assumption of the existence of this as the foun- 

 dation of all sedimentary rocks. 



It is argued by the adherents of the metamorphic theory of eruptive rocks, in 

 the first place, that granite cannot be the fundamental rock in the crust of the globe, 

 on which those of sedimentary origin are resting, since even very ancient masses of it 

 are frequently found to overlie stratified rocks. But this fact, which was known long 

 ago, shows only that it is impossible, on account of the great thickness of sedimentary 

 rocks, to know by ocular observation of what nature is the fundamental rock. It is 

 (112) 



