7A RICHTHOFEN— 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- 
low the sedimentary rocks, we have still to examine where is the limit to which the 
theory of such an origin may be 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, syenite 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 voleanic 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 
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