72 RICHTHOFEN NATURAL SYSTEM 



compounds. It is from this point of view, as we have repeatedly remarked, that are 

 offered the chief and most deeply-founded differences between the eruptive and the 

 stratified and foliated rocks. Definite numerical relations on the one side, complete ab- 

 sence of them on the other, and a transition between both, marked by the gradual dis- 

 appearance of those relations with the passage from granite, in which they are very 

 distinct, to gneiss and micaschist these are in short the prominent characters of the 

 great divisions of those rocks which are accessible to the observations of the geologist. 

 We alluded at another place to the corollary of this distinction, namely : that it is 

 impossible that eruptive rocks are remelted sediments, because, if they were, they 

 would necessarily have to participate in the varied and indefinite chemical composition 

 of these. This important argument appears to have been completely overlooked by 

 the adherents of the metamorphic doctrine, and it would be sufficient by itself to 

 make the latter appear to be in contradiction with the true state of facts. K 



If we descend from this general tie embracing the totality of eruptive rocks, 

 to those relations which either separate or connect distinct groups of them, in regard 

 to the time at which they came to the surface, we are unable to find any explanation 

 of the peculiar uniformity of these relations, if we adopt the metamorphic doctrine. 

 Rocks of great mutual similarity might have been occasionally ejected at different 

 places ; but the repetition of the same order of succession in distant regions would be 

 just as inexplicable, and contradictory to the nature of sedimentary rocks, as the per- 

 fect chemical identity of rocks which are widely separated in space, but occupy a 

 similar position in the order of succession. 



Another geological consideration may be mentioned which appears to weaken the 

 metamorphic theory. It is known that the most ancient rocks are distinguished, in 

 general, by a much greater similarity among each other in respect to chemical compo- 

 sition than is the case with those of more recent origin, and that one prominent feat- 

 ure of the majority of them is the presence of silica in a similar proportion to that in 

 which it is contained in granite. The formation of sedimentary rocks having been 

 due, at all times, to the disintegration of rocky matter antecedent to them in age, it is 

 obvious that ancient sediments would have participated in the silicious nature of the 



25 It can hardly be comprehended that it should have been maintained, and be believed, with our present state of knowl- 

 edge, that clayslutes were, by metamorphic action, converted into granite and syenite, and sandstones into porphyry ; since 

 granite and (quartzose) porphyry are chemically identical, while clayslatc and sandstone differ in this respect not only among 

 themselves, but each of them represents a large number of different accidental compounds, without any law of mutual con- 

 nection. The supposition that " the presence of the sandstone formations affords the conditions required for the occurrence 

 of the great porphyry-masses," gives evidence of an interpretation of natural occurrences by preconceived ideas. There are, it 

 is true, numerous instances of sandstones having acquired, by metamorphic action, a more or less perfect porphyritic texture, 

 so as to be hardly capable of being distinguished, in specimens, from true eruptive porphyries ; but geological observation 

 will seldom leave any doubt in regard to their true mode of occurrence ; and, as a generality, it is not difficult to distinguish 

 these metamorphic rocks with porphyritic texture from " the great porphyry-masses," such as those of southern Norway, or 

 middle Germany, to which the above-mentioned supposition has been especially applied. It is evident that the eruptions of 

 the porphyritic rocks were there subaqueous, and that the true relation is exactly the reverse of that suggested : the ejec- 

 tion of great masses of qnartzose porphyry afforded all the conditions required for the formation of sandstone beds. Quartzose 

 porphyry did overflow vast regions, and, by its immediate disintegration into tufaceous matter, gave origin to those red 

 sandstones by which it is so often accompanied. These cover the porphyry often in horizontal beds, and bear to it similar 

 relations of gradual passage and intcrstratification, as trachytic tufa does to trachyte, where the eruptions of the latter were 

 subaqueous. 



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